Magnetic disk substrate, magnetic disk, and magnetic disk device

ABSTRACT

It is an object to provide a magnetic disk substrate highly reliable to prevent the occurrence of crash failure even if a magnetic disk is rotated at high speed, and suitable for a hard disk that starts and stops by the load/unload method, and a magnetic disk using such a substrate. 
     The representative structure of a magnetic disk substrate according to this invention is a disk-shaped glass substrate  10  having a generally flat main surface  11,  an end face  12,  a chamfered face  13  formed between the main surface  11  and the end face  12,  and an offset portion, at the periphery of the main surface  11,  raised or lowered with respect to a flat surface, other than the periphery, of the main surface  11,  and characterized in that the magnitude of the offset portion is approximately uniform over the entire circumference of the glass substrate  10.

This is a divisional of application Ser. No. 12/527,818, filed Aug. 19,2009, which is a National Stage Application filed Under §371 of PCTApplication No. PCT/JP2008/052710, filed Feb. 19, 2008, which claimsforeign priority to JP 2007-038926, filed Feb. 20, 2007. The entiredisclosures of the prior applications, application Ser. No. 12/527,818,PCT/JP2008/052710, and JP 2007-038926 are considered part of theaccompanying divisional application and are hereby incorporated byreference in their entirety.

TECHNICAL FIELD

This invention relates to a magnetic disk substrate for use in amagnetic recording medium and to a magnetic disk using the same.

BACKGROUND ART

In recent years, the information recording technique, particularly themagnetic recording technique, has remarkably advanced following thedevelopment of information technology. As a magnetic recording medium,being one of magnetic recording media, to be mounted in an HDD (harddisk drive) or the like, there is a magnetic disk. The magnetic disk isformed by coating a film of NiP (nickel phosphorus) or the like on ametal substrate made of an aluminum-magnesium alloy or stacking anunderlayer, a magnetic layer, a protective layer, and a lubricatinglayer in this order on a substrate such as a glass substrate or aceramic substrate. Aluminum substrates have conventionally been widelyused as magnetic disk substrates. However, following the reduction insize and thickness and the increase in recording density of magneticdisks, glass substrates excellent in substrate surface flatness andsubstrate strength as compared with the aluminum substrates have beengradually replacing them.

The glass substrates with high rigidity are also advantageous in termsthat the improvement in impact resistance is also required for mountinglarge-capacity magnetic recording media in mobile devices andautomobiles. The size of substrates tends to be reduced for installationin mobile devices. Accordingly, starting from conventional 3.5-inchsubstrates, there have been required 2.5-inch substrates, 1.8-inchsubstrates, 1-inch substrates, and smaller substrates. As the size ofthe substrates decreases, the allowable dimensional error also decreasesand thus more accurate shape processing is required.

Further, following the increase in density of the magnetic recordingtechnique, magnetic heads have also shifted from thin film heads tomagnetoresistive heads (MR heads) and to giant magnetoresistive heads(GMR heads), wherein the flying height of a magnetic head from asubstrate has decreased to even 10 nm or less. However, when themagnetic head flies over a magnetic disk with such an extremely lowflying height, there is a problem that a fly stiction failure tends tooccur. The fly stiction failure is a failure in which a magnetic headflying over a magnetic disk causes abnormality in flying posture orflying height, which causes irregular reproduction output changes. Ifthis fly stiction failure occurs, there may occur a head crash failurein which the flying magnetic head is brought into contact with themagnetic disk. Therefore, the glass substrate surfaces have beenrequired to have high-level flatness and smoothness.

Further, for effectively using the area of the surface of the glasssubstrate, the load/unload type (Load Unload) has started to be employedin place of the conventional CSS type (Contact Start Stop). The CSS typeis a type in which a magnetic head is brought into contact with asubstrate surface at the time of disk stoppage, and thus it is necessaryto provide a CSS region (region for contact sliding with a magnetichead) on the substrate surface. In contrast, the load/unload type is atype in which a magnetic head is retreated to the outside of a glasssubstrate at the time of disk stoppage, and thus there is an advantagein that a CSS region can also be used as a recording surface. Further,during stoppage of a magnetic disk device, even if a strong impact isapplied, since the magnetic head is retreated, it is possible tominimize damage to a magnetic disk. For a portable small-sized harddisk, a combination of a start reproduction system of the load/unloadtype and a magnetic disk using a glass substrate is selected in terms ofensuring the information recording capacity and improving the impactresistance.

In the load/unload type, since a magnetic head passes through an endportion of a glass substrate, the shape at an outer peripheral portionof the glass substrate particularly arises as a problem. If there isdisturbance in shape (rising or lowering) at the outer peripheralportion of the glass substrate, the flying posture of the magnetic headis disturbed so that a contact tends to occur when the magnetic headcomes in from the outside of the glass substrate or goes out, and thusthere is a possibility of the occurrence of crash failure. Therefore,high flatness is required particularly at the disk outer peripheralportion.

Not only an increase in density but also an increase in speed isrequired for magnetic disks. Conventionally, a magnetic disk devicemounted with a glass substrate has used a relatively low rotationalspeed of 4200 rpm or the like. In recent years, however, a rotationalspeed of, for example, 7200 rpm or more has started to be used. Further,in near future, a rotational speed of 10000 rmp or more is expected tobe used. With such high-speed rotation, the linear velocity particularlynear the outer periphery of a magnetic disk increases. For example, in amagnetic disk at a rotational speed of 4200 rpm, the linear velocity ata position of radius 32.5 mm from the substrate center is 14.3 m/sec,while, the linear velocity becomes 18.4 m/sec at 5400 rpm and the linearvelocity becomes 24.5 m/sec at 7200 rpm. The above-mentioned flystiction failure and head crash failure particularly tend to occur atthe disk outer peripheral portion where the linear velocity becomes highas described above. Therefore, also in this viewpoint, high flatness isrequired particularly at the outer peripheral portion.

In recent years, the contact-sliding type recording medium(contact-recording type recording medium) has been re-evaluated. Thecontact-sliding type recording medium is a recording type in which arecording head reads and writes in a state where it is in slidingcontact with a magnetic disk. Although the contact-sliding typerecording medium itself is the recording type that has been present fora long time, since the recording density can be increased as thedistance between a recording head and a magnetic disk is reduced, it isagain considered to be the recording type that will be developed infuture. As the flying height of a recording head decreases, there is acase where the recording head contacts a magnetic disk. That is, as aresult of reducing the flying height of the recording head, there is acase where, partially, the recording head makes a sliding contact withthe magnetic disk. However, if it makes the sliding contact, wear of therecording head becomes a big problem. Further, there is also a problemthat if the recording head jumps, there is a possibility that the signalquality degrades or the recording head is damaged due to impact uponjumping or landing. These are all largely attributable to unevenness ofthe surface of the magnetic disk and, as the rotational speed (i.e. thelinear velocity) of the magnetic disk increases, the influenceincreases. Therefore, also in this viewpoint, high flatness is requiredparticularly at the outer peripheral portion.

On the other hand, as described in Patent Document 1 (JP-A-2005-141852),there has conventionally been a problem that when the main surface of asubstrate is polished, the flatness of its outer peripheral portionbecomes insufficient. That is, a glass substrate is polished by pressingthe front and back main surfaces thereof between polishing pads andrelatively moving the glass substrate and the polishing pads whilesupplying a slurry containing abrasives. In this event, rising (theouter peripheral portion of the main surface projects as compared withthe other portion of the main surface) called ski jump occurs at theouter peripheral portion of the main surface or lowering (the outerperipheral portion of the main surface falls in a state of being shavedrelatively greater than the other portion of the main surface) calledroll-off occurs at the outer peripheral portion of the main surface.Either one of the ski jump and the roll-off may occur or both may occur.

-   Patent Document 1: JP-A-2005-141852

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

As described above, the outer peripheral portion of the magnetic disk isa portion where the flatness is most required because the linearvelocity becomes highest and thus the influence of unevenness is large.Further, with respect to the passage of the magnetic head in theload/unload type, the flatness is also required at the outer peripheralportion of the magnetic disk. At the outer peripheral portion, the skijump or the roll-off tends to occur and thus the flatness tends todegrade. Therefore, it is necessary to reduce the ski jump or theroll-off as much as possible or to manage so that a glass substrate withreduced ski jump or roll-off is used for a magnetic disk. And, whenmanufacturing a magnetic disk substrate, this end portion shape is usedas one of indices for judging a good/defective product.

However, as a result of manufacturing a magnetic disk, i.e. a hard disk,using a magnetic disk substrate thus managed, there has arisen a problemof frequent occurrence of head crash.

Therefore, an attempt has been made to reduce the head crash by morestrictly setting a management value for defining the above-mentioned endportion shape (more strictly setting a judgment standard for agood/defective product based on the end portion shape). As a result, theratio of occurrence of head crash is relatively reduced, but still,there has arisen a problem of occurrence of head crash.

This invention has been made in view of the above-mentioned problems andhas an object to provide a substrate highly reliable to prevent theoccurrence of crash failure even if a magnetic disk is rotated at highspeed, and suitable for a hard disk that starts and stops by theload/unload method, and a magnetic disk using such a substrate.

Means for Solving the Problem

As a result of diligently studying the above-mentioned problems, theinventors of this application have paid attention to the fact that thereare cases where a head crash occurs and where no head crash occurs evenby strictly setting the above-mentioned management value, and haveobserved the end portion shape of respective substrates. Then, they havefound that there is variation in end portion shape in the in-plane ofthe glass substrate judged to have no problem in terms of the managementvalue.

First Mode of this Invention

Further, the inventors of this application have found that it ispossible to provide a glass substrate capable of preventing a head crashby suppressing variation in end portion shape in the in-plane of theglass substrate, more specifically, controlling the raised (lowered)shape at an end portion of a main surface of the glass substrate so asto be approximately the same in a height direction perpendicular to themain surface of the glass substrate, and have completed the first modeof this invention.

That is, a magnetic disk substrate according to the first mode of thisinvention has a structure in which the magnetic disk substrate is adisk-shaped substrate and has a generally flat main surface, an endface, a chamfered face formed between the main surface and the end face,and an offset portion, at a periphery of the main surface, raised orlowered with respect to a flat surface, other than the periphery, of themain surface, wherein a magnitude of offset of the offset portion isapproximately uniform over the entire circumference of the substrate.

In other words, the structure is characterized in that the main surfaceof the substrate has, between itself and the chamfered face, the offsetportion raised or lowered with respect to the main surface and, in planview of the main surface of the substrate, the offset portion surroundsthe main surface with an approximately uniform height. Herein, “anapproximately uniform height” is such that, for example, the differencein height of the offset portion is preferably 5 nm or less.

According to the above-mentioned structure, it is possible to improvethe flatness of particularly the outer peripheral portion of themagnetic disk substrate. Therefore, the flight posture of a magnetichead is prevented from being disturbed particularly at the outerperipheral portion of a magnetic disk and, even if the magnetic disk isrotated at high speed, there is no possibility of contact between themagnetic disk and the magnetic head and thus the reliability can beenhanced. Further, also with respect to passage of a magnetic head inthe load/unload type, there is no possibility that the flight posture ofthe magnetic head is disturbed particularly at the outer peripheralportion of a magnetic disk or that the magnetic head is brought intocontact with the magnetic disk.

That is, by causing the height of the raised shape (rising) formed atthe end portion of the main surface of the substrate over the entirecircumference of the substrate to be approximately the same in thecircumferential direction of the substrate, when a magnetic disk device(hard disk drive: HDD) is manufactured using this substrate, it ispossible to stabilize the flight of a magnetic head and thus to preventthe occurrence of head crash.

It is preferable that the magnitude of the offset be approximatelyuniform in the circumferential direction at arbitrary radial positions.Since a magnetic head scans a magnetic disk mainly in thecircumferential direction, contact between the magnetic disk and themagnetic head can be more prevented by setting the magnitude of theoffset to be approximately uniform in the circumferential direction.

When measuring the offset portion of the substrate at 12 points per 30°in the circumferential direction, the change in magnitude of the offset,i.e. the difference (change amount), in a direction perpendicular to themain surface of the substrate, in the offset portion formed along thecircumference of the substrate, is preferably 5 nm or less. With thechange in this range, the effect of the first mode of this invention canbe obtained more reliably.

An extreme portion where rising or lowering is maximum in the offsetportion is preferably located at approximately the same distance fromthe center of the substrate. In other words, the substrate has acircular hole in its center and it is preferable that, in the mainsurface, the center of a circle formed by an extreme portion whererising or lowering is maximum in the offset portion be located atapproximately the same position as the center of the above-mentionedcircular hole. This enables the magnitude of the offset to beapproximately uniform in the circumferential direction.

In the main surface, the circularity of a circle formed by an extremeportion where rising or lowering is maximum in the offset portion ispreferably 600 μm or less. If 400 μm or less, it is more preferable.Further, ideally, it is preferably 200 μm or less. If the circularitydecreases (value increases), even if the magnitude of the offset of theextreme portion is uniform, the magnitude of the offset cannot beapproximately uniform as seen in the circumferential direction. Bysetting the circularity to be within the above-mentioned range, however,the magnitude of the offset in the circumferential direction can be madeapproximately uniform with respect to the size of a recording head.

The substrate has a circular hole in its center and, in the mainsurface, the concentricity between a circle formed by an extreme portionwhere rising or lowering is maximum in the offset portion and thecircular hole is preferably 1200 μm or less. If 1000 μm or less, it ismore preferable. Further, ideally, it is preferably 800 μm or less. Ifthe concentricity decreases (value increases), even if the offset of theextreme portion is uniform, the magnitude of the offset cannot beapproximately uniform as seen in the circumferential direction. Bysetting the concentricity to be within the above-mentioned range,however, the magnitude of the offset in the circumferential directioncan be made approximately uniform with respect to the size of arecording head.

When seen in a direction perpendicular to the main surface, the offsetportion is preferably 0.02 mm+1.00 mm (width of a pico-slider of arecording head)=1.02 mm taking into account the circularity of a circlealong the offset portion and the size of the recording head with respectto the disk radial direction. In other words, when seen from a sectionof the glass substrate, the change in position of the offset portionpreferably falls within a range of 1.02 mm or less in the radialdirection of the substrate.

The substrate may be a substrate for use in a magnetic disk to bemounted in a magnetic disk device of a load/unload type in which amagnetic head is loaded and unloaded with respect to a main surface ofthe magnetic disk through its outer periphery. Since the flatness of theouter peripheral portion is high, the substrate can be suitable for theload/unload type.

The substrate may be a substrate for use in a magnetic disk to bemounted in a magnetic disk device adapted to rotate the magnetic disk ata rotational speed of at least 5400 rpm. Since the flatness of the outerperipheral portion is high, even if the magnetic disk is rotated at highspeed, there is no possibility of contact between the magnetic disk anda magnetic head and thus the reliability can be enhanced.

The representative structure of a magnetic disk manufacturing methodaccording to the first mode of this invention is characterized byforming at least a magnetic layer on a surface of a magnetic disksubstrate obtained by the above-mentioned magnetic disk substratemanufacturing method. This makes it possible to manufacture a magneticdisk having high-level flatness also at an outer peripheral portion of amain surface thereof.

Second Mode of this Invention

The inventors of this application have found that it is possible toprovide a glass substrate capable of preventing a head crash bysuppressing variation in end portion shape in the in-plane of the glasssubstrate, more specifically, controlling the raised shape at an endportion of a main surface of the glass substrate so as to beapproximately the same in the radial direction of the glass substrate,and have completed the second mode of this invention.

That is, in order to solve the problem, a magnetic disk substrateaccording to the second mode of this invention has a structure in whichthe magnetic disk substrate is a disk-shaped substrate and has agenerally flat main surface, an end face, a chamfered face formedbetween the main surface and the end face, and an offset portion, at aperiphery of the main surface, raised with respect to a flat surface,other than the periphery, of the main surface, wherein the offsetportion is formed over the entire circumference of the substrate, and anextreme portion where rising is maximum in the offset portion is locatedat approximately the same distance from a center of the substrate.

According to the above-mentioned structure, it is possible to improvethe flatness, in the circumferential direction being a scanningdirection of a recording head, particularly at the outer peripheralportion of the magnetic disk substrate. Therefore, the flight posture ofthe magnetic head is prevented from being disturbed particularly at theouter peripheral portion of a magnetic disk and, even if the magneticdisk is rotated at high speed, there is no possibility of contactbetween the magnetic disk and the magnetic head and thus the reliabilitycan be enhanced. Further, also with respect to passage of a magnetichead in the load/unload type, there is no possibility that the flightposture of the magnetic head is disturbed particularly at the outerperipheral portion of a magnetic disk or that the magnetic head isbrought into contact with the magnetic disk.

That is, by causing the radial position of the raised shape (rising)formed at the end portion of the main surface of the substrate over theentire circumference of the substrate to be approximately the same inthe circumferential direction of the substrate, when a magnetic diskdevice (hard disk drive: HDD) is manufactured using this substrate, itis possible to stabilize the flight of a magnetic head and thus toprevent the occurrence of head crash.

In the main surface, the extreme portion of the raised offset portion ispreferably located over the entire circumference of the offset portionin a range of 92.0 to 97.0% with respect to the radial distance from thecenter of the substrate to its outer diameter. It may alternatively bewithin a range of 1 to 2.6 mm from the outer peripheral end face of thesubstrate as a reference. This makes it possible to know the position ofthe extreme portion by conventional measurement of ski jump, roll-off,dub-off, or the like.

In the main surface, the circularity of a circle formed by the extremeportion is preferably 600 μm or less. If 400 μm or less, it is morepreferable. Further, ideally, it is preferably 200 μm or less. If thecircularity decreases (value increases), even if the magnitude of theoffset of the extreme portion is uniform, the magnitude of the offsetcannot be approximately uniform as seen in the circumferentialdirection. By setting the circularity to be within the above-mentionedrange, however, the magnitude of the offset in the circumferentialdirection can be made approximately uniform with respect to the size ofa recording head.

The substrate has a circular hole in its center and, in the mainsurface, the concentricity between a circle formed by the extremeportion and the circular hole is preferably 1200 μm or less. If 1000 μmor less, it is more preferable. Further, ideally, it is preferably 800μm or less. If the concentricity decreases (value increases), even ifthe offset of the extreme portion is uniform, the magnitude of theoffset cannot be approximately uniform as seen in the circumferentialdirection. By setting the concentricity to be within the above-mentionedrange, however, the magnitude of the offset in the circumferentialdirection can be made approximately uniform with respect to the size ofa recording head.

The magnitude of the offset in the offset portion may be approximatelyuniform over the entire circumference of the substrate. That is, themain surface of the substrate has, between itself and the chamferedface, the offset portion raised or lowered with respect to the mainsurface and, in plan view of the main surface of the substrate, theoffset portion may surround the main surface with an approximatelyuniform height. It is preferable that the magnitude of the offset beapproximately uniform in the circumferential direction at arbitraryradial positions. Since a magnetic head scans a magnetic disk mainly inthe circumferential direction, contact between the magnetic disk and themagnetic head can be more prevented by setting the magnitude of theoffset to be approximately uniform in the circumferential direction.Herein, “approximately uniform” is preferably 0.02 mm+1.00 mm (width ofa pico-slider of a recording head)=1.02 mm taking into account both thecircularity being in the above-mentioned range and the size of therecording head with respect to the disk radial direction.

In the offset portion, the change in magnitude of the offset, i.e. thedifference (change amount), in a direction perpendicular to the mainsurface of the substrate, in the offset portion formed along thecircumference of the substrate, is preferably 5 nm or less. With thechange in this range, the effect of the second mode of this inventioncan be obtained more reliably.

The substrate may be a substrate for use in a magnetic disk to bemounted in a magnetic disk device of a load/unload type in which amagnetic head is loaded and unloaded with respect to a main surface ofthe magnetic disk through its outer periphery. Since the flatness of theouter peripheral portion is high, the substrate can be suitable for theload/unload type.

The substrate may be a substrate for use in a magnetic disk to bemounted in a magnetic disk device adapted to rotate the magnetic disk ata rotational speed of at least 5400 rpm or more. Since the flatness ofthe outer peripheral portion is high, even if the magnetic disk isrotated at high speed, there is no possibility,of contact between themagnetic disk and a magnetic head and thus the reliability can beenhanced.

The representative structure of a magnetic disk manufacturing methodaccording to the second mode of this invention is characterized byforming at least a magnetic layer on a surface of a magnetic disksubstrate obtained by the above-mentioned magnetic disk substratemanufacturing method. This makes it possible to manufacture a magneticdisk having high-level flatness also at an outer peripheral portion of amain surface thereof.

Effect of the Invention

According to the first mode of this invention, it is possible to improvethe flatness of particularly the outer peripheral portion of a magneticdisk substrate. Therefore, the flight posture of a magnetic head isprevented from being disturbed particularly at the outer peripheralportion of a magnetic disk and, even if the magnetic disk is rotated athigh speed, there is no possibility of contact between the magnetic diskand the magnetic head and thus the reliability can be enhanced. Further,also with respect to passage of a magnetic head in the load/unload type,there is no possibility that the flight posture of the magnetic head isdisturbed particularly at the outer peripheral portion of a magneticdisk or that the magnetic head is brought into contact with the magneticdisk.

According to the second mode of this invention, it is possible toimprove the flatness, in the circumferential direction being a scanningdirection of a recording head, particularly at the outer peripheralportion of a magnetic disk substrate. Therefore, the flight posture ofthe magnetic head is prevented from being disturbed particularly at theouter peripheral portion of a magnetic disk and, even if the magneticdisk is rotated at high speed, there is no possibility of contactbetween the magnetic disk and the magnetic head and thus the reliabilitycan be enhanced. Further, also with respect to passage of a magnetichead in the load/unload type, there is no possibility that the flightposture of the magnetic head is disturbed particularly at the outerperipheral portion of a magnetic disk or that the magnetic head isbrought into contact with the magnetic disk.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a diagram used for explaining a feature of this inventionand is a diagram explaining the case where the end portion shape of amagnetic disk substrate is a ski-jump shape, and FIG. 1( b) is a diagramused for explaining a feature of this invention and is a diagramexplaining the case where the end portion shape of a magnetic disksubstrate is a roll-off shape.

FIG. 2 is a diagram used for explaining a feature of this invention andis a diagram explaining a maximum offset value from a straight lineconnecting two arbitrary points in the end portion shape of a magneticdisk substrate.

FIG. 3 is a diagram used for explaining a first embodiment of thisinvention and is a diagram showing the results of measuring the maximumoffset value from a straight line connecting two arbitrary points in theend portion shape of a magnetic disk substrate.

FIG. 4 is a diagram used for explaining a first embodiment of thisinvention and is a diagram showing the results of measuring the radialposition of an extreme portion when the end portion shape of a magneticdisk substrate is a ski-jump shape.

FIG. 5 is a diagram used for explaining a first embodiment of thisinvention and is a diagram showing the results of measuring the maximumoffset value from a straight line connecting two arbitrary points in theend portion shape of a magnetic disk substrate.

FIG. 6 is a diagram used for explaining a second embodiment of thisinvention and is a diagram showing the results of measuring the radialposition of an extreme portion when the end portion shape of a magneticdisk substrate is a ski-jump shape.

FIG. 7 is a diagram used for explaining a second embodiment of thisinvention and is a diagram showing the results of measuring the maximumoffset value from a straight line connecting two arbitrary points in theend portion shape of a magnetic disk substrate.

DESCRIPTION OF SYMBOLS

10 glass substrate

11 main surface

12 end face

13 chamfered face

14 offset portion

15 extreme portion

BEST MODE FOR CARRYING OUT THE INVENTION

Next, a description will be given of embodiments each of a magnetic disksubstrate according to this invention and a magnetic disk using it.Sizes, materials, specific numerical values, and so on shown in thefollowing embodiments are only illustrative for facilitatingunderstanding of the invention and are not intended to limit thisinvention unless otherwise specified.

First, referring to FIGS. 1 and 2, a description will be given of an“offset portion”, an “extreme portion”, and so on that will be used forexplaining a feature of this invention.

FIG. 1 shows side views for explaining a ski-jump shape and a roll-offshape each being one example of the end portion shape of a magnetic disksubstrate.

The above-mentioned magnetic disk substrate 10 has a disk shape and isformed with a circular hole in its center. As shown in FIG. 1, themagnetic disk substrate 10 has main surfaces 11 that will serve asinformation recording/reproducing regions, an end face 12 perpendicularto the main surfaces 11, and chamfered faces 13 interposed between themain surfaces and the end face, respectively. Since it may happen thatthe boundary between the end face 12 and each chamfered face 13 becomesunclear by a later-described end face polishing process, this inventionalso includes a case where the end face 12 and the chamfered faces 13 onboth sides thereof cooperatively form one curved surface.

Since each main surface is the region for recording/reproducinginformation, it is substantially flat for allowing a recording head tofly thereover. However, in the manufacture of the glass substrate 10, anoffset portion 14 that is, for example, raised or lowered with respectto the main surface of the glass substrate as compared with a centralportion of the main surface is formed at the periphery of the mainsurface. This offset portion 14 is formed on each of the innerperipheral end portion side and the outer peripheral end portion side ofthe main surface of the glass substrate.

As shown in FIG. 1( a), the ski-jump shape is a shape such that an outerperipheral portion of the main surface 11 is raised, while, as shown inFIG. 1( b), the roll-off shape is a shape such that an outer peripheralportion of the glass substrate 10 is lowered.

FIG. 2 shows an example of performing a measurement using a maximumoffset value from a straight line connecting two arbitrary points. Thatis, an evaluation is implemented by using the straight line connectingthe two arbitrary points as a reference plane and measuring themagnitude of an extreme portion 15 (maximum offset value) being amaximum point of rising or lowering as seen from the reference plane.

As shown in FIG. 2, the maximum offset value between two points is amaximum distance in the positive direction from a straight line,connecting two arbitrary points R1 and R2, within that range. And it ispossible to measure a state of the extreme portion 15 by setting thepositions of the two points R1 and R2 so that the extreme portion 15being a vertex of the ski-jump shape is included and the maximumdistance from the straight line connecting the two points is located onthe extreme portion 15 side.

Specifically, when setting the above-mentioned two arbitrary points R1and R2, in the case of, for example, a substrate having an outerdiameter size of 2.5 inches (outer diameter 65 mmφ), it is possible todetermine the distances from the center of the glass substrate to be29.9 mm (R1) and 31.5 mm (R2), respectively. In other words, assumingthat the distance from the center of the substrate to an end facethereof is 100%, it is also possible to determine the two arbitrarypoints as two points at a position of 92% and a position of 97% from thecenter of the substrate. And the offset portion (raised portion orlowered portion) 14 of the magnetic disk substrate exists between thesetwo points. Of these two points, the point further from the center ofthe substrate is a region over which a magnetic head flies.

In the embodiments described hereinbelow, rising or lowering formed withrespect to the flat surface other than the periphery of the main surfaceat the periphery thereof as described above will be generally referredto as “offset” with respect to the flat surface of the main surface. Anda portion formed with this offset will be referred to as the offsetportion 14. As seen by referring to FIGS. 1 and 2, the positionexhibiting the maximum offset is the extreme portion 15.

First Embodiment

First, the first embodiment of this invention will be described.

(Magnetic Disk Substrate)

As a result of diligent studies for reducing unevenness of amagnetic-disk main surface due to ski jump or roll-off to therebyprovide a magnetic disk that can prevent the occurrence of crash failureeven if rotated at high speed, the inventors have found that the stateof ski jump or roll-off differs at different positions in thecircumferential direction even in the same substrate, and have completedthe first mode of this invention described before.

That is, also conventionally, it has been judged before shipping a glasssubstrate whether the glass substrate is good or not by measuring theshape of a peripheral portion. However, since glass substrates should bemanufactured at low cost and in large quantities and since inspectedsubstrates cannot be used as shipping products (destructive test), somesamples are extracted from a lot and each of them is measured at only asingle position thereof. And it has been found that even if magneticdisks are manufactured using the glass substrates judged to be good as aresult of the measurement, there are cases where the number of glasssubstrates judged to be defective by a glide test is large and where thenumber of glass substrates judged to be defective by a glide test issmall.

In view of this, the inventors have made a detailed examination and havefound that when the end portion shape is measured at a plurality ofpositions in the circumferential direction, the position and magnitudeof an extreme portion 15 differ depending on the position. It has beenfound that, for that reason, even in the case of a lot having passed aninspection, when a glass substrate is actually formed into a magneticdisk and incorporated into a magnetic disk device, there is a case wherethe desired performance cannot be exhibited. On the other hand, it hasbeen found that when a glass substrate in which the magnitude of anextreme portion 15 formed at the periphery of a main surface of theglass substrate, in other words, the maximum value of ski jump (rising)formed at an end portion of the main surface, is approximately the samein the circumferential direction is formed into a magnetic disk andincorporated into a magnetic disk device, no head crash occurs.

Therefore, in this embodiment, the offset portion 14 formed with theoffset is formed over the entire circumference of the glass substrate 10and, further, the magnitude of the offset being a maximum value ofrising or lowering in the offset portion 14 is made approximatelyuniform over the entire circumference of the glass substrate 10. Inother words, the main surface 11 of the glass substrate 10 has, betweenitself and the chamfered face, the offset portion 14 raised or loweredwith respect to the main surface and, in plan view of the main surfaceof the glass substrate, the offset portion 14 surrounds the main surfacewith an approximately uniform height. This will be describedhereinbelow.

Next, a description will be given of a manufacturing method formanufacturing the magnetic disk substrate according to this embodiment.

A magnetic disk substrate is manufactured through various processes(details will be described later), but when manufacturing the magneticdisk substrate according to this embodiment having the extreme portion15 being the vertex of the offset portion (raised portion or loweredportion) 14 at the peripheral portion of the main surface atapproximately the same distance from the center of the glass substrate,particularly a final polishing process (second polishing process)becomes important. Whether the raised portion is formed at theperipheral portion of the main surface or whether the lowered portionlowered with respect to the flat surface is formed at the peripheralportion of the main surface is also mostly determined by the polishingconditions of the final polishing process. In the following description,the conditions for forming the raised portion will be described.

The required height of the raised portion has been much more reducedfollowing improvement in storage density and most of factors thatdetermine the shape/magnitude thereof depend on the polishing conditionsof the final polishing process.

Many of various polishing conditions in the final polishing processaffect the shape/magnitude of the raised portion, but among them,particularly the processing rate (processing speed) and the processingpressure affect them.

Hereinbelow, a description will be given of the final polishing processthat polishes main surfaces of a glass substrate using a planetary geartype polishing machine. It is needless to say that the final polishingprocess can be carried out without using the planetary gear typepolishing machine. For example, the final polishing process may beapplied to the glass substrate using a single wafer type polishingmachine.

In the final polishing process, the glass substrate is polished byrelatively moving polishing pads and the glass substrate while pressingthe polishing pads onto both main surfaces of the glass substrate. Inthis event, the machining amount per unit time is the processing rateand the pressure pressing the glass substrate is the processingpressure.

For manufacturing the magnetic disk substrate according to thisembodiment, it is preferable that the processing rate be set in a rangeof 0.20 μm/min to 0.45 μm/min and the processing pressure be set in arange of 8.0 Pa to 10.5 Pa. The other polishing conditions haverelatively small influences and thus are not limitative, but in the caseof, for example, a 2.5-inch disk (φ65 mm), it is possible to set thehardness of the polishing pads to 85 (Asker C hardness) and the grainsize of abrasives to 1.0(μm). If polishing not satisfying theabove-mentioned conditions is carried out, there is a possibility ofdegradation of the end portion shape or breakage of the glass substrate.

For manufacturing the magnetic disk substrate according to thisembodiment, it is preferable that, after polishing the substrate at theprocessing pressure (main processing pressure) intended for thepolishing processing, the substrate be polished at a processing pressure(e.g. 1 Pa or less) lower than the main processing pressure in the finalpolishing process. Particularly, it is preferable that the polishing becarried out at this low processing pressure for a time about half thepolishing time of polishing the substrate at the main processingpressure. This makes it possible to reduce variation in end portionshape in the circumferential direction.

For manufacturing the magnetic disk substrate according to thisembodiment, it is preferable to obtain a magnetic disk glass substrateby, after applying a chemical strengthening treatment to a glasssubstrate capable of being chemically strengthened, polishing mainsurfaces of the substrate. When the chemical strengthening treatment(ion-exchange treatment) is applied, there is a case where the endportion shape becomes rougher than that after the polishing. Thus, interms of reducing variation in end portion shape in the circumferentialdirection, by applying the polishing treatment after applying thechemical strengthening treatment, it is possible to produce, at highyield, magnetic disk glass substrates with reduced variation in endportion shape in the circumferential direction. The glass substratesubjected to the main surface polishing treatment after the chemicalstrengthening treatment is a glass substrate having an ion-exchangedlayer at at least a part of the substrate surfaces and faces, whereinthe thickness of the ion-exchanged layer is greater at the end facesthan at the main surfaces.

The glass substrate subjected to the main surface polishing treatmentafter the chemical strengthening treatment is also preferable in termsof reducing the roughness of the main surfaces. Particularly, thesubstrate surface roughness required for the recent perpendicularmagnetic recording system has been significantly decreasing thanconventional. In order to satisfy this requirement, it is preferablethat the glass substrate be subjected to the main surface polishingtreatment after the chemical strengthening treatment.

The surface roughness Ra of the magnetic disk substrate according tothis embodiment, measured using an AFM (electron microscope), ispreferably 0.15 nm or less.

For manufacturing the magnetic disk substrate according to thisembodiment, when performing final polishing using a planetary gear typepolishing machine, the relationship between the on-its-axis rotationspeed of a carrier and the orbital rotation speed of the carrierorbiting in the machine also becomes important.

In the planetary gear type, a plurality of glass substrates are held inthe carrier. Then, polishing pads are pressed onto upper and lowersurfaces of the glass substrates held in the carrier. Then, in thisstate, the glass substrates are polished when the carrier orbits whilerotating on its axis. That is, the directions of the relative movementof the glass substrates and the polishing pads are random and thus areaveraged, but have no relationship at all with the circumferentialdirection of the glass substrates. Thus, ski jump or the like formed atthe outer peripheral portion of each glass substrate has variation inradial position and magnitude at positions in the circumferentialdirection.

In order to reduce this variation in the circumferential direction, itis preferable to set the ratio between the on-its-axis rotation speedand the orbital rotation speed of the carrier within a range of 0.125 to8. If polishing is performed under a condition exceeding this range, theshape of ski jump (raised portion) formed at the periphery of each glasssubstrate is often disturbed in the circumferential direction.

Then, if the magnitude (height) of the offset varies in thecircumferential direction when seeing the glass substrate in a directionperpendicular to the main surface of the glass substrate, severevertical changes occur when a magnetic disk rotates and a recording head(magnetic head) makes scanning, so that the flight of the recording headbecomes unstable. If the recording head cannot follow the change inmagnitude of the offset, a head crash occurs.

To explain this in detail, when a recording head flies over a magneticdisk, even if changes in magnitude of offset are large, if the ratio ofthe changes is small (gentle), the recording head can follow them.However, if there is a large offset change in a very short flightdistance, a head crash occurs. That is, the magnetic disk substrateaccording to this embodiment is a particularly preferable mode when thehead flies at high speed.

On the other hand, by setting the magnitude of the offset of the offsetportion 14 to be approximately uniform over the entire circumference ofthe glass substrate as the above-mentioned structure, even if the skijump or roll-off is present, as long as the recording head constantlypasses over the ridge of the extreme portion 15 during scanning, it isconsidered that there is no height difference large enough to cause acrash failure. That is, this invention pays attention to the fact thatit is the change in height of the substrate surface in thecircumferential direction that is important.

Therefore, according to the structure of this embodiment, it is possibleto reduce the roughness or undulation in the circumferential directionand thus to improve the flatness particularly at the outer peripheralportion of the magnetic disk substrate. Therefore, despite the ski jumpor roll-off is present, the flight posture of the magnetic head isprevented from being disturbed and, even if the magnetic disk is rotatedat high speed, there is no possibility of contact between the magneticdisk and the magnetic head. This makes it possible to significantlyenhance the reliability when the magnetic disk is incorporated into amagnetic disk device. Further, also with respect to passage of amagnetic head in the load/unload type, since there is no possibilitythat the flight posture of the magnetic head is disturbed particularlyat the outer peripheral portion of a magnetic disk or that the magnetichead is brought into contact with the magnetic disk, the recording headcan stably pass through the end portion of the magnetic disk, which isquite preferable.

It is preferable that the magnitude of the offset be approximatelyuniform in the circumferential direction at arbitrary radial positions.Since a magnetic head scans a magnetic disk mainly in thecircumferential direction, contact between the magnetic disk and themagnetic head can be more prevented by setting the magnitude of theoffset to be approximately uniform in the circumferential direction.

Herein, “approximately uniform in the circumferential direction” ispreferably 0.02 mm+1.00 mm (width of a pico-slider of a recordinghead)=1.02 mm taking into account the circularity being in theabove-mentioned range and the size of the recording head with respect tothe disk radial direction.

Specifically, for example, when measuring the offset portion 14 of theglass substrate at 12 points per 30° in the circumferential direction,the change in magnitude of the offset can be 5 nm or less. With thechange in this range, the effect of this invention can be obtained morereliably.

Further, the extreme, portion 15 where rising or lowering is maximum inthe offset portion 14 is preferably located at approximately the samedistance from the center of the glass substrate. This is because even ifthe magnitude of the offset is approximately uniform over the entirecircumference, if there is disturbance in distance from the center, itcannot be approximately uniform in the circumferential direction.

Specifically, in the main surface, the circularity of a circle formed bythe extreme portion 15 where rising or lowering is maximum in the offsetportion 14 is preferably 600 μm or less. If 400 μm or less, it is morepreferable. Further, ideally, it is preferably 200 μm or less. If thecircularity decreases (value increases), even if the magnitude of theoffset of the extreme portion 15 is uniform, the magnitude of the offsetcannot be approximately uniform as seen in the circumferentialdirection. In other words, when seeing a section at a predeterminedradial position (position where the offset portion 14 exists) from thecenter of the glass substrate, if the glass substrate is rotated, themagnitude of the extreme portion 15 of the offset portion 14 (e.g. theheight of the raised portion) changes. That is, if the recording headcannot follow a change in magnitude of the offset portion 14 whileflying over the predetermined radial position, a head crash occurs. Bysetting the circularity to be within the above-mentioned range, however,the magnitude of the offset in the circumferential direction can be madeapproximately uniform with respect to the size of the recording head:

Further, the concentricity between the circular hole formed in thecenter of the glass substrate and the circle formed by the extremeportion 15 where rising or lowering is maximum in the offset portion 14is preferably 1200 μm or less. If 1000 μm or less, it is morepreferable. Further, ideally, it is preferably 800 μm or less. If theconcentricity decreases (value increases), even if the magnitude of theoffset of the extreme portion 15 is uniform, the magnitude of the offsetcannot be approximately uniform as seen in the circumferentialdirection. By setting the concentricity to be within the above-mentionedrange, however, the magnitude of the offset in the circumferentialdirection can be made approximately uniform with respect to the size ofthe recording head.

The magnetic disk substrate according to this embodiment is preferablyused as a glass substrate to be mounted in a magnetic disk deviceadapted to rotate it at a rotational speed of 5400 rpm or more. Thereis, of course, no problem even if it is mounted in a magnetic diskdevice adapted to rotate it at a rotational speed of 5400 rpm or more,but particularly in the case of a high-rotational-speed magnetic diskdevice, the effect of using the magnetic disk substrate according tothis embodiment remarkably appears as compared with that of using amagnetic disk substrate of other than this invention.

The magnetic disk substrate according to this embodiment is preferablyused as a glass substrate to be mounted in a magnetic disk device inwhich a magnetic head scans an offset portion 14 present at theperiphery of the glass substrate at a linear velocity of 20.0 m/sec ormore.

The magnetic disk substrate according to this embodiment is preferablyused for a magnetic disk with a touch-down height (TDH) of 3 to 4 nm orless. If the touch-down height is low, a crash tends to occur when thechange of offset formed on the glass substrate is large. However, usingthe magnetic disk substrate of this embodiment, the change of an offsetportion 14 can be further reduced than conventional and, thus, even ifthe flying height of a magnetic head (recording head) is reduced, it ispossible to prevent the magnetic head from being crashed.

The magnetic disk substrate according to this embodiment is preferablyused for a magnetic disk having a high recording density of 200GBit/inch² or more and more preferably 250 GBit/inch² or more. In thecase of such a high recording density, the flying height of a recordinghead needs to be further reduced, but since the flying height of therecording head can be further reduced by the use of the magnetic disksubstrate of this embodiment, a crash can be prevented. It is needlessto say that the magnetic disk substrate according to this invention canalso be suitably applied to a glass substrate for use in a magnetic diskhaving a recording density smaller than the above or to a magnetic disksubstrate for manufacturing a magnetic disk with a touch-down heightgreater than the above.

The magnetic disk substrate according to this embodiment is adisk-shaped magnetic disk substrate having a flat main surface, an endface, and a chamfered face interposed between the main surface and theend face, wherein, at a peripheral portion of the main surface, thereexists a raised portion raised in a direction perpendicular to a flatsurface, excluding the peripheral portion, of the main surface andwherein, in plan view of the main surface of the glass substrate, theraised portion surrounds the main surface and the raised portion maysurround the main surface with an approximately uniform height.

In plan view of the main surface of the glass substrate, the maximumpoint of an offset portion 14 may be present in a range of 92.0 to 97.0%with respect to the distance to the external of the glass substrate fromthe center of the glass substrate as a reference.

In plan view of the main surface of the glass substrate, an extremeportion 15 of the raised portion circularly surrounds the main surfaceand the circularity of a circle formed by the extreme portion 15 ispreferably 600 μm or less.

The glass substrate has a circular hole at its central portion and, inplan view of the main surface of the glass substrate, an extreme portion15 of the raised portion circularly surrounds the main surface and theconcentricity between a circle formed by the extreme portion 15 and acircle formed by the circular hole at the central portion is preferably1200 μm or less.

The magnetic disk substrate according to this embodiment is adisk-shaped glass substrate having a flat main surface, an end face, anda chamfered face interposed between the main surface and the end face,wherein, in plan view of the main surface of the glass substrate, alowered portion lowered with respect to the main surface is providedbetween the main surface and the chamfered face and the depth of thelowered portion from the main surface may be approximately uniform atpositions spaced from the center of the glass substrate by apredetermined distance.

In the magnetic disk substrate according to this embodiment, it ispreferable that at least the end face be chemically strengthened and itis more preferable that all the surfaces of the substrate be chemicallystrengthened. In other words, in the magnetic disk substrate, it is morepreferable that a compressive stress layer be formed at the surfacesthereof. Particularly, when the magnetic disk substrate is incorporatedinto a magnetic disk device adapted to rotate at high-speed rotation(e.g. 10000 rpm) or into a magnetic disk device for use in a mobileapplication, the impact resistance is required for the glass substrateand thus it is preferable that the compressive stress layer be formed onthe surfaces of the glass substrate. Herein, the chemical strengtheningis a treatment for strengthening a glass substrate by bringing the glasssubstrate into contact with a chemical strengthening treatment solutioncontaining a chemical strengthening salt to thereby exchange part ofions contained in the glass substrate for ions in the chemicalstrengthening treatment solution having an ionic radius greater thanthat of the former ions.

(Magnetic Disk)

Then, by forming a magnetic film on the above-mentioned magnetic disksubstrate, it is possible to manufacture a magnetic disk according tothis embodiment. Since the magnetic film is formed on the magnetic disksubstrate, the shape of a main surface of the magnetic disk is largelyaffected by the glass substrate. That is, in order to improve the shapeof the main surface of the magnetic disk, it is necessary to improve theshape of the main surface of the glass substrate. Therefore, using themagnetic disk substrate disclosed in this embodiment, it is possible tomanufacture the magnetic disk improved particularly in shape of aperipheral portion of the main surface. Since a magnetic diskmanufacturing method is known, a description thereof is omitted herein.

The magnetic disk according to this embodiment may be a magnetic disk tobe mounted in a magnetic disk device of the load/unload type in which amagnetic head is loaded and unloaded with respect to the main surfacethrough its outer periphery. Since the flatness of the outer peripheralportion of the glass substrate is high, the magnetic disk can besuitable for the load/unload type.

The magnetic disk according to this embodiment may be a magnetic disk tobe mounted in a magnetic disk device adapted to rotate it at arotational speed of at least 5400 rpm or more. Further, it can besuitably used even in a magnetic disk device with a speed of 7200 rpm ormore or 10000 rpm or more. This is because since the flatness of theouter peripheral portion of the glass substrate is high, there is nopossibility of contact between the magnetic disk and a magnetic headeven in the case of high-speed rotation and thus the reliability ishigh.

The magnetic disk according to this embodiment may be a contact-slidingtype recording medium (contact-recording type recording medium). In thecase of the contact-sliding type recording medium, a recording headreads and writes in a state where it is in sliding contact with amagnetic disk and, therefore, by improving the flatness, in thecircumferential direction, of particularly the outer peripheral portionof the magnetic disk substrate as described above, it is possible toprevent jumping of the recording head. This makes it possible to improvethe signal quality and to prevent damage to the recording head.

(Magnetic Disk Device)

Then, by mounting the above-mentioned magnetic disk, it is possible toform a magnetic disk device (hard disk drive). The magnetic disk devicemounted with the above-mentioned magnetic disk substrate is particularlysuitable when recording/reproducing information particularly athigh-speed rotation.

Although the description is being given centering on the outerperipheral end face of the magnetic disk substrate, an offset portion 14is also formed with respect to the inner peripheral end face in themanner as described above. It is needless to say that an extreme portion15 where rising in the offset portion 14 is maximum is preferablylocated at approximately the same distance from the center of the glasssubstrate.

(Method of Managing Manufacture of Magnetic Disk Glass Substrate)

In the foregoing description, the description has been given of theconfiguration in which the state of the entire offset of a glasssubstrate is evaluated by measuring dub-off, ski jump, or the like at aplurality of different positions on the glass substrate. However, themeasurement of the dub-off, ski jump, or the like takes a long time and,currently, about five minutes are required for one position. Thus, ifmeasuring 12 points as in the above-mentioned embodiment, one hour isrequired for one substrate. Since glass substrates should bemass-produced at low cost, it is preferable to omit useless measurementas much as possible.

In view of this, when actually manufacturing a magnetic disk glasssubstrate, variation of offset (rising or lowering) on the substratesurface may be measured only when the maximum value of the offset on thesubstrate surface is a predetermined value or less, thereby judging agood/defective product. More specifically, a measurement process ofmeasuring the magnitude of offset may be divided into a positionmeasurement process of measuring a position where offset (rising orlowering) is maximum and a value measurement process of measuring thedetailed magnitude of offset. It is considered that the positionmeasurement process is, for example, a measurement method with aresolution lower than that of the value measurement process and performsa measurement quickly and in a wide range. In this position measurementprocess, although the resolution is low, the magnitude of the offset canbe measured with certain level accuracy. Therefore, when the magnitudeof the offset clearly exceeds the range of good products, it is notnecessary to carry out a detailed measurement for that substrate. On theother hand, when the magnitude of the offset falls within a rangecombining the range of good products and an error range of the positionmeasurement process, 12 points may be measured in the value measurementprocess as in the above-mentioned embodiment.

Specifically, the position measurement process entirely scans a glasssubstrate using, for example, OptiFlat (manufactured by Phase ShiftTechnology, Inc.) as a low-resolution optical interference surface shapemeasuring apparatus and is capable of specifying a position where themagnitude of offset is maximum. The value measurement process canmeasure the detailed magnitude of offset using, for example, MicroXam(manufactured by the same company) as a high-resolution opticalinterference surface shape measuring apparatus.

With the configuration described above, it is possible to excludeapparently useless measurement and thus to reduce the time required forthe inspection process.

Further, it may alternatively be configured that, after grasping theposition where the magnitude of the offset of the glass substrate ismaximum in the position measurement process, the magnitude of the offsetis measured only at that position, i.e. at only the single point, in thevalue measurement process.

Then, in a judgment process, it is possible to judge whether the glasssubstrate is good or not by comparing the measured maximum magnitude ofthe offset with a predetermined value. When using dub-off as an offsetvalue, the offset value is ±10 nm or less, preferably ±7 nm or less, andmore preferably ±5 nm or less. The offset value measurement range can beset to a range of 92.0 to 97.0% from the center of a glass substrateassuming that the distance from the center of the glass substrate to anend portion thereof is set to 100%.

As described above, by measuring only the largest offset of thesubstrate, it is not necessary to perform a measurement at a pluralityof positions. This makes it possible to make a quick judgment and thusto achieve a reduction in time of the inspection process.

It has been described that the substrate according to this invention isa glass (amorphous glass or glass ceramic (crystallized glass) can beused; as a material of a plate-like glass, aluminosilicate glass,sodalime glass, borosilicate glass, or the like). However, since thisinvention relates to the shape of a magnetic disk substrate, thisinvention is not limited to a material of the magnetic disk substrateand thus can be suitably applied, for example, even to a substrate madeof aluminum or a material other than it. As described above, however, aglass substrate excellent in substrate surface flatness and substratestrength as compared with an aluminum substrate is preferableparticularly for a mobile device.

The magnetic disk substrate according to this invention is a magneticdisk substrate having a generally flat main surface, an end face, and achamfered face formed between the main surface and the end face and maybe configured such that, at the periphery of the main surface, there isprovided an offset portion 14 raised or lowered with respect to a flatsurface, other than the periphery, of the main surface, the offsetportion 14 is continuously formed in the circumferential direction ofthe main surface, and the maximum height (maximum offset value) of theoffset portion 14 in a direction perpendicular to the main surface isapproximately uniform over the entire circumference of the substrate.

The offset portion 14 may be formed at the outer periphery or the innerperiphery of a main surface of a disk-shaped magnetic disk formed withan inner hole in its center or may be formed at each of them. In thisinvention, when the offset portion 14 is formed at each of the innerperiphery and the outer periphery, at least one of the offset portions14 may have the above-mentioned shape, but it is preferable thatparticularly the outer periphery where the linear velocity of arecording head is high have the above-mentioned shape.

By manufacturing a magnetic disk using the magnetic disk substrate ofthis embodiment, there can be provided the magnetic disk with smallvariation in touch-down height (TDH) in the radial direction of thesubstrate. By reducing variation in end portion shape of the substrateas described above, it is possible to improve the flying characteristicsof a magnetic head near the outermost periphery of the disk.

The magnetic disk substrate according to this embodiment is preferablyused as a substrate of a magnetic disk adapted for a DFH (dynamic flyingheight) head. In the case of using the DFH head, the distance betweenthe surface of the magnetic disk and the closest portion of the head ismuch shorter than conventional. However, in the magnetic disk substrateof this embodiment, the height of undulation can be set to 20 μm or lessand further to 12 μm or less. The surface roughness of the main surfaceof the substrate can be set to 0.15 nm or less and further to 0.12 nm orless. Thus, when the above-mentioned magnetic disk substrate is used fora magnetic disk, it is possible to further reduce a crash of a DFH head.

EXAMPLE

Hereinbelow, a description will be given of an Example about methods ofmanufacturing a magnetic disk substrate and a magnetic disk to whichthis invention is applied. These magnetic disk substrate and magneticdisk are manufactured as a magnetic disk having a predetermined shape,such as a 3.5-inch disk (φ89 mm), a 2.5-inch disk (φ65 mm), a 0.8-inchdisk (φ21.6 mm), a 1.0-inch disk (φ27.4 mm), or a 1.8-inch magnetic disk(φ48 mm).

(1) Shaping Process and First Lapping Process

In the magnetic disk substrate manufacturing method according to thisExample, first, lapping (grinding) is applied to surfaces of aplate-like glass to obtain a glass base member, then this glass basemember is cut, thereby cutting out a glass disk. As the plate-likeglass, one of various plate-like glasses can be used. This plate-likeglass can be manufactured, for example, by a known manufacturing method,such as a pressing method, a float method, a downdraw method, aredrawing method, or a fusion method, using a molten glass as amaterial. Among them, if the pressing method is used, the plate-likeglass can be manufactured at low cost. As a material property of theplate-like glass, use can be made of an amorphous glass or a glassceramic (crystallized glass). As a material of the plate-like glass, usecan be made of an aluminosilicate glass, a sodalime glass, aborosilicate glass, or the like. Particularly as the amorphous glass,the aluminosilicate glass can be preferably used in terms of capabilityof being chemically strengthened and capability of providing a magneticdisk substrate excellent in main surface flatness and substratestrength.

In this Example, a molten aluminosilicate glass was molded into a diskshape by direct pressing using upper, lower, and drum molds, therebyobtaining an amorphous plate-like glass. As the aluminosilicate glass,use was made of a glass for chemical strengthening which contains, asmain components, SiO₂: 58 to 75 wt %, Al₂O₃: 5 to 23 wt %, Li₂O: 3 to 10wt %, and Na₂O: 4 to 13 wt %.

Then, lapping was applied to both main surfaces of the plate-like glass,thereby obtaining a disk-shaped glass base member. The lapping wascarried out using a double-side lapping machine employing a planetarygear mechanism with the use of alumina-based free abrasive grains.Specifically, the lapping was carried out by pressing lapping surfaceplates onto both surfaces of the plate-like glass from the upper andlower sides, supplying a grinding liquid containing the free abrasivegrains onto the main surfaces of the plate-like glass, and relativelymoving the plate-like glass and the surface plates to each other. Bythis lapping, there was obtained the glass base member having the flatmain surfaces.

(2) Cutting-Out Process (Coring, Forming, Chamfering)

Then, the glass base member was cut using a diamond cutter, therebycutting out a disk-shaped glass substrate from the glass base member.Then, using a cylindrical diamond drill, an inner hole was formed at acenter portion of the glass substrate, thereby obtaining an annularglass substrate (coring). Then, grinding was applied to an innerperipheral end face and an outer peripheral end face using diamondgrindstones, thereby carrying out predetermined chamfering (forming,chamfering).

(3) Second Lapping Process

Then, second lapping was applied to both main surfaces of the obtainedglass substrate in the same manner as in the first lapping process. Byperforming this second lapping process, fine irregularities formed onthe main surfaces in the cutting-out process or an end face polishingprocess as a preceding process can be removed in advance, so that itbecomes possible to complete a subsequent main surface polishing processin a short time.

(4) End Face Polishing Process

Then, the outer and inner peripheral end faces of the glass substratewere mirror-polished by a brush polishing method. In this event, aspolishing abrasive grains, use was made of a slurry (free abrasivegrains) containing cerium oxide abrasive grains.

Then, the glass substrate having been subjected to the end facepolishing process was washed with water. By this end face polishingprocess, the end faces of the glass substrate were finished to a mirrorsurface state that can prevent precipitation of sodium and potassium.

(5) Main Surface Polishing Process

A first polishing process was first carried out as a main surfacepolishing process. This first polishing process mainly aims to removecracks or strains remaining on the main surfaces during theabove-mentioned lapping processes. In this first polishing process, themain surfaces were polished using a double-side polishing machine havinga planetary gear mechanism with the use of a hard resin polisher. Ceriumoxide abrasive grains were used as a polishing liquid.

The glass substrate having been subjected to the first polishing processwas immersed in respective cleaning baths of neutral detergent, purewater, and IPA (isopropyl alcohol) in turn so as to be cleaned.

Then, a second polishing process was carried out as a main surfacepolishing process. This second polishing process aims to finish the mainsurfaces to a mirror surface state. In this second polishing process,the main surfaces were mirror-polished using a double-side polishingmachine having a planetary gear mechanism with the use of a soft resinfoam polisher. As a polishing liquid, use was made of cerium oxideabrasive grains finer than the cerium oxide abrasive grains used in thefirst polishing process.

The glass substrate having been subjected to the second polishingprocess was immersed in respective cleaning baths of neutral detergent,pure water, and IPA (isopropyl alcohol) in turn so as to be cleaned. Anultrasonic wave was applied to each cleaning bath.

(6) Chemical Strengthening Process

Then, chemical strengthening was applied to the glass substrate havingbeen subjected to the above-mentioned lapping processes and polishingprocesses. The chemical strengthening was carried out by preparing achemical strengthening solution in the form of a mixture of potassiumnitrate (60%) and sodium nitrate (40%), heating this chemicalstrengthening solution to 400° C. and preheating the cleaned glasssubstrate to 300° C., and immersing it in the chemical strengtheningsolution for about 3 hours. The immersion was carried out in a statewhere a plurality of glass substrates were placed in a holder so as tobe held at their end faces, thereby enabling the entire surfaces of theglass substrates to be chemically strengthened.

By performing the immersion treatment in the chemical strengtheningsolution as described above, lithium ions and sodium ions in a surfacelayer of the glass substrate are replaced by sodium ions and potassiumions in the chemical strengthening solution, respectively, so that theglass substrate is strengthened. The thickness of a compressive stresslayer formed at the surface layer of the glass substrate was about 100μm to 200 μm.

The glass substrate having been subjected to the chemical strengtheningtreatment was immersed in a water bath at 20° C. so as to be rapidlycooled, and maintained for about 10 minutes. Then, the rapidly cooledglass substrate was immersed in concentrated sulfuric acid heated toabout 40° C., so as to be cleaned. Further, the glass substrate havingbeen subjected to the sulfuric acid cleaning was immersed in respectivecleaning baths of pure water and IPA in turn so as to be cleaned.

By performing the first lapping process, the cutting-out process, thesecond lapping process, the end face polishing process, the first andsecond polishing processes, and the chemical strengthening process asdescribed above, there was obtained a flat, smooth, and high-rigiditymagnetic-disk substrate.

(7) Inspection Process

An inspection was conducted about the shape of an outer peripheralportion of the obtained magnetic disk substrate. An inspection processcomprises a measurement process of measuring offset values and anextreme portion 15 at a plurality of positions in the circumferentialdirection of the glass substrate and a judgment process of judgingwhether the glass substrate is good or not based on the measured offsetvalues and extreme portion 15.

(8) Magnetic Disk Manufacturing Process

On each of both surfaces of the glass substrate obtained through theabove-mentioned processes, an adhesive layer of a Cr alloy, a softmagnetic layer of a CoTaZr-group alloy, an underlayer of Ru, aperpendicular magnetic recording layer of a CoCrPt-group alloy, aprotective layer of hydrocarbon, and a lubricating layer ofperfluoropolyether were formed in this order, thereby manufacturing aperpendicular magnetic recording disk. More specifically, an adhesivelayer of CrTi, a soft magnetic layer of CoTaZr/Ru/CoTaZr, anintermediate layer of Ru, a granular magnetic layer of CoCrPt—SiO₂, anda hydrogenated carbon protective layer were formed in this order on aglass substrate using an in-line type sputtering apparatus and further aperfluoropolyether lubricating layer was formed by a dipping method,thereby obtaining a magnetic disk.

This structure is one example of the structure of a perpendicularmagnetic disk (PMR: Perpendicular Magnetic Recording), while, magneticlayers and so on may be formed as a horizontal magnetic disk (LMR:Longitudinal Magnetic Recording). This makes it possible to manufacturea magnetic disk having high-level flatness also at an outer peripheralportion of a main surface thereof.

(9) Magnetic Disk Device Manufacturing Process

By incorporating the above-mentioned magnetic disk into a device, amagnetic disk device was manufactured. Since the structure of a magneticdisk device is known, a detailed description thereof is omitted herein.

Example 1

By applying the following polishing conditions to the second polishingprocess of (5) Main Surface Polishing Process described above, amagnetic disk substrate, a magnetic disk, and a magnetic disk devicewere manufactured. In this Example 1, a 2.5-inch disk (φ65 mm) wasmanufactured. The specific polishing conditions were such that thehardness of polishing pads was set to 85 (Asker C hardness), the grainsize of abrasives to 1.0(μm), the processing rate to 0.30(μm/min), andthe processing pressure to 9(Pa). More specifically, the processingpressure in the final polishing process was changed in two stages sothat, after performing polishing at a main processing pressure of 9(Pa)for a predetermined time, polishing was performed at a processingpressure of 1(Pa) for a time half the predetermined time. In this event,the product of the main processing pressure and the processing rate(main processing pressure×processing rate) was 2.7.

Comparative Example 1

A magnetic disk substrate, a magnetic disk, and a magnetic disk deviceaccording to Comparative Example 1 were manufactured by theabove-mentioned manufacturing method except that the polishingconditions of the second polishing process were set to the followingconditions. The specific polishing conditions in Comparative Example 1were such that the hardness of polishing pads was set to 85 (Asker Chardness), the grain size of abrasives to 1.0(μm), the processing rateto 0.60(μm/min), and the processing pressure to 12.0(Pa). In thispolishing process, polishing was carried out while maintaining a mainprocessing pressure of 12.0(Pa) without dropping the processing pressurethereafter. In this event, the product of the main processing pressureand the processing rate (main processing pressure×processing rate) was7.2.

Comparison between Example 1 and Comparative Example 1

The shape of an extreme portion 15 present at the periphery of a mainsurface of each of the magnetic disk substrates manufactured as shown inExample 1 and Comparative Example 1 was inspected by the followingmethod.

(A) Influence of End Portion Shape

First, the influence of the height of the extreme portion 15 in thecircumferential direction was examined. Specifically, in order toexamine the change in height of each glass substrate in thecircumferential direction, the maximum offset value of each substratebetween two arbitrary points was measured. The measurement range (R1, R2in FIG. 2) of the maximum offset value was determined so that it waspossible to see the change of the extreme portion 15 formed at theperiphery of the main surface of the glass substrate and having heightsthat differ in the circumferential direction. Herein, the distances fromthe center of the substrate were set to 29.9 mm (R1) and 31.5 mm (R2),respectively, and, as a measuring apparatus, use was made of an opticalinterference surface shape measuring apparatus (MicroXam (manufacturedby Phase Shift Technology, Inc.): objective lens magnification; 2.5times, intermediate lens magnification; 0.62 times, measurementwavelength; 553.2 nm, measurement region; 3.58×3.88 mm, resolution;752×480 pixels). Then, the position of the vertex was measured at 12points in total by rotating the glass substrate per 30° in thecircumferential direction.

That is, the maximum offset value measured by the above-mentioned methodrepresents the magnitude of the offset between the above-mentionedstraight line connecting R1 and R2 and the vertex (extreme portion 15)of the above-mentioned raised portion. The results are shown in FIG. 3.

As shown in FIG. 3, the change in height of the glass substrate in thecircumferential direction is small in Example 1 as compared with that inComparative Example 1. Specifically, the change difference in Example 1was 2.86 nm, while, the change difference in Comparative Example 1 was16.10 nm.

(B) Load/Unload Test Comparison

As described above, after manufacturing the magnetic disks in which amagnetic layer was formed on the magnetic disk substrates according toExample 1 and Comparative Example 1, the magnetic disk devices weremanufactured, and a load/unload test was conducted. Specifically, thetest was carried out in two cases with a disk rotational speed of 5400rpm and 7200 rpm by setting the flying height of a recording head to 9to 10 nm.

As a result, in the case of the magnetic disks according to Example 1and Comparative Example 1, no crash occurred even by repeating theload/unload 1000000 times at a rotational speed of 5400 rpm. Withrespect to the magnetic disk of Comparative Example 1, a crash occurredin a load/unload test of 2000000 times.

On the other hand, when a load/unload test was performed by setting therotational speed to 7200 rpm, while no crash occurred even by repeatingthe load/unload 1000000 times in the case of the magnetic disk accordingto Example 1, a crash occurred in the case of the magnetic diskaccording to Comparative Example 1 when the load/unload was repeated800000 times.

From this result, it is seen to be important that, as in this invention,the magnitude of the offset of the extreme portion (vertex of the raisedportion) 15 of the offset portion 14 present at the periphery of themain surface of the glass substrate be approximately uniform over theentire circumference of the glass substrate.

Although the description is being given using the vertex of the raisedportion as the extreme portion 15 of the offset portion 14 in thisExample, this invention can alternatively use a valley point of alowered portion as an extreme portion 15 of an offset portion 14. Thatis, the same effect as above can be obtained by causing the magnitude ofoffset of the extreme portion 15, where lowering is maximum, to beapproximately uniform over the entire circumference of a glasssubstrate.

(C) Specification of Surface Shape

Next, the shape (end portion shape) of the periphery of the main surfaceof the magnetic disk substrate was examined. An examination was made ofthe influence of the concentricity and circularity of positions of theextreme portion (vertex of the raised portion) 15 of the magnetic disksubstrate in which the magnitude of the offset of the offset portion 14present at the periphery of the main surface of the glass substrate isapproximately uniform over the entire circumference of the glasssubstrate (Example 1 described above). Further, another magnetic disksubstrate in which the concentricity and circularity of positions of anextreme portion (vertex of a raised portion) 15 are lower than those ofthe above-mentioned Example 1 was prepared as Comparative Example 2.This magnetic disk substrate of Comparative Example 2 was manufacturedby setting the processing pressure and the processing rate in the finalpolishing process to be different from those in the above-mentionedExample 1. Specifically, the magnetic disk substrate of ComparativeExample 2 was manufactured by setting the processing pressure to 8.0(Pa)and the processing rate to 0.45(μm/min) for the glass substrate. In thisevent, the product of the main processing pressure and the processingrate (main processing pressure×processing rate) was 3.6.

An optical interference surface shape measuring apparatus (OptiFlat(manufactured by Phase Shift Technology, Inc.)) was used for themeasurement. OptiFlat is low in resolution but wide in measurement rangeas compared with MicroXam described above.

As a result, it was found that the end portion shape of the glasssubstrates of Example 1 and Comparative Example 2 was a ski-jump shape.And from this measurement result, the distance of the vertex (extremeportion 15) in the ski-jump shape from the center of the glass substratewas measured. Further, in order to examine the displacement of theposition of the vertex in the circumferential direction, the position ofthe vertex was measured at 12 points in total by rotating the glasssubstrate per 30° in the circumferential direction. The results in thisevent are shown in FIG. 4. FIG. 4 is a diagram showing the results ofmeasuring the radial position of the extreme portion (ski-jump point) 15of the ski jump.

As a result, as shown in FIG. 4, it was found that the extreme portion15 formed at the periphery of the main surface of the magnetic disksubstrate according to Example 1 was located at approximately the sameposition (distance) as seen from the center of the glass substrate and,specifically, was located in a range of ±0.2 mm with respect to 30.6 mmfrom the center of the glass substrate. On the other hand, in the caseof Comparative Example 2, it was found that the extreme portion 15 waslocated in a range of ±1.4 mm with respect to 30.6 mm.

The circularity of the magnetic disk substrate according to Example 1was 0.40(mm) and the concentricity thereof was 1.07(mm). On the otherhand, the circularity of the magnetic disk substrate according toComparative Example 2 was 2.60(mm) and the concentricity thereof was5.68(mm). With respect to the height (offset value) of the extremeportion 15, the values were approximately the same in the structures ofExample 1 and Comparative Example 2.

Then, for the above-mentioned Example 1 and Comparative Example 2, aload/unload test was conducted in which the rotational speed was set to10000 rpm. This test was carried out in a state of a magnetic disk. Inthis event, the flying height of a recording head was set to 9 to 10 nm.As a result, no crash occurred even by repeating the load/unload 1000000times in the case of Example 1, while, in the case of ComparativeExample 2, a crash occurred when the load/unload was repeated 600000times. In Example 1, in the case of a rotational speed of 5400 rpm and7200 rpm, no crash occurred even by repeating the load/unload 1000000times.

Further, despite the offset values in the magnetic disk substrates wereapproximately the same in Example 1 and Comparative Example 2, a crashfailure occurred with the structure of Comparative Example 2 when aglide test was carried out at high-speed rotation. On the other hand, nocrash failure occurred with the structure of Example 1. This ispresumably because even if large (not small) ski jump is present, sincethe circularity or concentricity is high (value is small), the change(roughness or undulation) of the substrate surface in thecircumferential direction is small. Further, as a result of many tests,it was found that the circularity was preferably 600 μm or less and theconcentricity was preferably 1200 μm or less.

From this result, it is seen to be most preferable in terms of rotatingthe magnetic disk at high speed that the extreme portion 15 be the samein radial position and small in height change in the circumferentialdirection of the glass substrate.

(D) Modulation Test

A modulation test was conducted for the magnetic disks obtained inExample 1 and Comparative Example 1. Specifically, a modulation wasmeasured in a region between distances of 29.9 mm (R1) and 31.5 mm (R2)from the center of the 2.5-inch (outer diameter 65 mmφ) glass substrate.

Specific measurement conditions followed the following sequence (1) to(3).

(1) Set a magnetic disk in an electromagnetic conversion characteristicmeasuring apparatus (Guzik Technical Enterprise) and, after loading amagnetic head (DFH (dynamic flying height) head) on the magnetic disk,write a MF pattern (frequency half a high frequency used for a harddisk).

(2) Input a read signal into an oscilloscope.

(3) Derive a modulation per sector at an arbitrary radial positionwithin the above-mentioned range.

As a result, in comparison between Example 1 and Comparative Example 1,modulation values were better in Example 1.

Further, the same modulation test as above was conducted using magneticdisk substrates having the same variation in maximum offset value asExample 1 (substrate in which the height of a raised portion is uniformin the circumferential direction), but having different circularity andconcentricity from each other. As a result, modulation values wereexcellent for those in which the circularity was 1200 μm or less and theconcentricity was 600 μm or less, while, if the circularity andconcentricity deviate from these values, the modulation results weredegraded. In Example 1 and Comparative Example 1, the magnetic diskswere manufactured under the same conditions.

Example 2

FIG. 5 is a diagram showing the results of measuring the maximum offsetvalue from a straight line connecting two arbitrary points with respectto Example 2 and Comparative Example 3. In Example 2, the processing wasperformed under the same conditions as in the above-mentioned Example 1and, in Comparative Example 3, the processing was performed under thesame conditions as in the above-mentioned Comparative Example 1. Forcomparison, FIG. 5 further shows Example 1 and Comparative Example 1.Like the results shown in FIG. 3, the position of the vertex (extremeportion 15) in a ski-jump shape was measured at 12 points in total byrotating a glass substrate per 30° in the circumferential direction.

As shown in FIG. 5, in Example 2, the maximum offset values are higheron the whole than those in Example 1, but the change (maximum-minimum)in height in the circumferential direction of the glass substrate wasstill smaller than that in Example 1. On the other hand, in ComparativeExample 3, the change was smaller than that in Comparative Example 1,but as seen from a graph, the maximum offset values irregularly go upand down. In general, the change differences in Example 1 and Example 2were as small as 2.86 nm and 1.95 nm, respectively, while, the changedifferences in Comparative Example 1 and Comparative Example 3 were aslarge as 16.10 nm and 12.50 nm, respectively.

The above-mentioned Examples and Comparative Examples relate to the casewhere the end portion shape is a ski-jump shape (shape raised withrespect to the main surface). Also in the case of a roll-off shape beinga shape lowered with respect to the substrate main surface, the sameresults as those of the above-mentioned Examples and ComparativeExamples were obtained as a result of carrying out the same tests.

While the first preferred embodiment of this invention has beendescribed with reference to the accompanying drawings, it is needless tosay that this invention is not limited thereto. It is obvious that aperson skilled in the art can think of various modified examples orrevised examples within the scope described in claims and it isunderstood that those naturally also belong to the technical scope ofthis invention.

Second Embodiment

Next, the second embodiment of this invention will be described.

(Magnetic Disk Substrate)

As a result of diligent studies for reducing unevenness of amagnetic-disk main surface due to ski jump or roll-off to therebyprovide a magnetic disk that can prevent the occurrence of crash failureeven if rotated at high speed, the inventors have found that the stateof ski jump or roll-off differs at different positions in thecircumferential direction even in the same substrate, and have alsocompleted the second mode of this invention described before.

That is, also conventionally, it has been judged before shipping a glasssubstrate whether the glass substrate is good or not by measuring theshape of a peripheral portion. However, since glass substrates should bemanufactured at low cost and in large quantities and since inspectedsubstrates cannot be used as shipping products (destructive test), somesamples are extracted from a lot and each of them is measured at only asingle position thereof. And it has been found that even if magneticdisks are manufactured using the glass substrates judged to be good as aresult of the measurement, there are cases where the number of glasssubstrates judged to be defective by a glide test is large and where thenumber of glass substrates judged to be defective by a glide test issmall.

In view of this, the inventors have made a detailed examination and havefound that when the end portion shape is measured at a plurality ofpositions in the circumferential direction, the position and magnitudeof an extreme portion 15 differ depending on the position. It has beenfound that, for that reason, even in the case of a lot having passed aninspection, when a glass substrate is actually formed into a magneticdisk and incorporated into a magnetic disk device, there is a case wherethe desired performance cannot be exhibited. On the other hand, it hasbeen found that when a glass substrate in which the radial position ofan extreme portion formed at the periphery of a main surface of theglass substrate, in other words, the radial position where the maximumvalue of ski jump (rising) formed at an end portion of the main surfaceexists, is approximately the same in the circumferential direction isformed into a magnetic disk and incorporated into a magnetic diskdevice, no head crash occurs.

Therefore, in this embodiment, it is configured that the offset portion14 formed with the offset is formed over the entire circumference of theglass substrate 10 and, further, the extreme portion 15 where rising ismaximum in the offset portion 14 is located at approximately the samedistance from the center of the glass substrate. This will be describedhereinbelow.

Next, a description will be given of a manufacturing method formanufacturing the magnetic disk substrate according to this embodiment.

A magnetic disk substrate is manufactured through various processes(details will be described later), but when manufacturing the magneticdisk substrate according to this embodiment having the extreme portionbeing the vertex of the offset portion (raised portion or loweredportion) 14 at the peripheral portion of the main surface atapproximately the same distance from the center of the glass substrate,particularly a final polishing process (second polishing process)becomes important. Whether the raised portion is formed at theperipheral portion of the main surface or whether the lowered portionlowered with respect to the flat surface is formed at the peripheralportion of the main surface is also mostly determined by the polishingconditions of the final polishing process. In the following description,the conditions for forming the raised portion will be described.

The required height of the raised portion has been much more reducedfollowing improvement in storage density and most of factors thatdetermine the shape/magnitude thereof depend on the polishing conditionsof the final polishing process.

Many of various polishing conditions in the final polishing processaffect the shape/magnitude of the raised portion, but among them,particularly the processing rate (processing speed) and the processingpressure affect them.

Hereinbelow, a description will be given of the final polishing processthat polishes main surfaces of a glass substrate using a planetary geartype polishing machine. It is needless to say that the final polishingprocess can be carried out without using the planetary gear typepolishing machine. For example, the final polishing process may beapplied to the glass substrate using a single wafer type polishingmachine.

In the final polishing process, the glass substrate is polished byrelatively moving polishing pads and the glass substrate while pressingthe polishing pads onto both main surfaces of the glass substrate. Inthis event, the machining amount per unit time is the processing rateand the pressure pressing the glass substrate is the processingpressure.

For manufacturing the magnetic disk substrate according to thisembodiment, it is preferable that the processing rate be set in a rangeof 0.25 to 0.5 μm/min and the processing pressure be set in a range of8.5 to 11 Pa. The other polishing conditions have relatively smallinfluences and thus are not limitative, but in the case of, for example,a 2.5-inch disk (φ65 mm), it is possible to set the hardness of thepolishing pads to 85 (Asker C hardness) and the grain size of abrasivesto 1.0(μm). If polishing not satisfying the above-mentioned conditionsis carried out, there is a possibility of degradation of the end portionshape or breakage of the glass substrate.

For manufacturing the magnetic disk substrate according to thisembodiment, when performing final polishing using a planetary gear typepolishing machine, the relationship between the on-its-axis rotationspeed of a carrier and the orbital rotation speed of the carrierorbiting in the machine also becomes important.

In the planetary gear type, a plurality of glass substrates are held inthe carrier. Then, polishing pads are pressed onto upper and lowersurfaces of the glass substrates held in the carrier. Then, in thisstate, the glass substrates are polished when the carrier orbits whilerotating on its axis. That is, the directions of the relative movementof the glass substrates and the polishing pads are random and thus areaveraged, but have no relationship at all with the circumferentialdirection of the glass substrates. Thus, ski jump or the like formed atthe outer peripheral portion of each glass substrate has variation inradial position and magnitude at positions in the circumferentialdirection.

In order to reduce this variation in the circumferential direction, itis preferable to set the ratio between the on-its-axis rotation speedand the orbital rotation speed of the carrier within a range of 0.125 to8. If polishing is performed under a condition exceeding this range, theshape of ski jump (raised portion) formed at the periphery of each glasssubstrate is often disturbed in the circumferential direction.

Then, if the radial position where the extreme portion is formed variesin the circumferential direction when seeing the glass substrate in adirection perpendicular to the main surface of the glass substrate,specifically, for example, if a circle formed by a line connectingportions of the extreme portion becomes eccentric or elliptic ormeanders, severe vertical changes occur when the circumferentialdirection (track direction) in which a recording head (magnetic head)makes scanning crosses a locus formed by portions of the extremeportion. In other words, when the recording head scans a magnetic diskrotating at high speed, the recording head passes across a plurality ofextreme portions and thus the flight of the recording head becomesunstable.

Following the reduction in flying height of recording heads in recentyears, there is a possibility that the recording head cannot follow suchvertical changes, leading to the occurrence of crash failure. Further,in the case of a contact-sliding type hard disk, there is a possibilitythat a recording head jumps to cause degradation of the signal qualityor damage to the recording head.

On the other hand, by causing the extreme portion to be located atapproximately the same distance from the center of the glass substrateas the above-mentioned structure, even if the ski jump or roll-off ispresent, as long as the recording head constantly passes over the ridgeof the extreme portion during scanning, it is considered that there isno height difference large enough to cause a crash failure. It is, ofcourse, expected that the extreme portion itself has a height differencedepending on the position in the circumferential direction, but thisheight difference is far smaller than rising or lowering of the extremeportion due to meandering or the like. That is, this invention paysattention to the fact that it is the change in height of the substratesurface in the circumferential direction that is important.

Therefore, according to the structure of this embodiment, it is possibleto reduce the roughness or undulation in the circumferential directionand thus to improve the flatness particularly at the outer peripheralportion of the magnetic disk substrate. Therefore, despite the ski jumpor roll-off is present, the flight posture of the magnetic head isprevented from being disturbed and, even if the magnetic disk is rotatedat high speed, there is no possibility of contact between the magneticdisk and the magnetic head. This makes it possible to significantlyenhance the reliability when the magnetic disk is incorporated into amagnetic disk device. Further, also with respect to passage of amagnetic head in the load/unload type, since there is no possibilitythat the flight posture of the magnetic head is disturbed particularlyat the outer peripheral portion of a magnetic disk or that the magnetichead is brought into contact with the magnetic disk, the recording headcan stably pass through the end portion of the magnetic disk, which isquite preferable.

Specifically, in the main surface, the circularity of a circle formed bythe extreme portion where rising or lowering is maximum in the offsetportion 14 is preferably 600 μm or less. If 400 μm or less, it is morepreferable. Further, ideally, it is preferably 200 μm. If thecircularity decreases (value increases), even if the magnitude of theoffset of the extreme portion is uniform, the magnitude of the offsetcannot be approximately uniform as seen in the circumferentialdirection. In other words, when seeing a section at a predeterminedradial position (position where the offset portion 14 exists) from thecenter of the glass substrate, if the glass substrate is rotated, themagnitude of the extreme portion of the offset portion 14 (e.g. theheight of the raised portion) changes. That is, if the recording headcannot follow a change in magnitude of the offset portion 14 whileflying over the predetermined radial position, a head crash occurs. Bysetting the circularity to be within the above-mentioned range, however,the offset value in the circumferential direction can be madeapproximately uniform with respect to the size of the recording head.

Further, the concentricity between the circular hole formed in thecenter of the glass substrate and the circle formed by the extremeportion where rising or lowering is maximum in the offset portion 14 ispreferably 1200 μm or less. If 1000 μm or less, it is more preferable.Further, ideally, it is preferably 800 μm or less. If the concentricitydecreases (value increases), even if the magnitude of the offset of theextreme portion is uniform, the magnitude of the offset cannot beapproximately uniform as seen in the circumferential direction. Bysetting the concentricity to be within the above-mentioned range,however, the magnitude of the offset in the circumferential directioncan be made approximately uniform with respect to the size of therecording head.

Further, it is preferable that the magnitude of the offset beapproximately uniform in the circumferential direction at arbitraryradial positions. By causing the circumferential direction to coincidewith the locus described by the extreme portion and by causing themagnitude of the offset of the extreme portion to be approximatelyuniform, contact between the magnetic disk and the magnetic head can bemore prevented. Herein, “approximately uniform” is preferably 0.02mm+1.00 mm (width of a pico-slider of a recording head)=1.02 mm takinginto account both the circularity being in the above-mentioned range andthe size of the recording head with respect to the disk radialdirection.

Specifically, for example, when measuring the offset portion 14 of themagnetic disk substrate at 12 points per 30° in the circumferentialdirection, the change in magnitude of the offset can be 5 nm or less.With the change in this range, the effect of this invention can beobtained more reliably.

As described above, if the recording head cannot follow the change inmagnitude of the offset, a head crash occurs. To explain this in detail,when a recording head flies over a magnetic disk, even if changes inmagnitude of offset are large, if the ratio of the changes is small(gentle), the recording head can follow them. However, if there is alarge offset change in a very short flight distance, a head crashoccurs. That is, the magnetic disk substrate according to thisembodiment is a particularly preferable mode when the head flies at highspeed. More specifically, the magnetic disk substrate according to thisembodiment is preferably used as a glass substrate to be mounted in amagnetic disk device adapted to rotate it at a rotational speed of 5400rpm or more. There is, of course, no problem even if it is mounted in amagnetic disk device adapted to rotate it at a rotational speed of 5400rpm or more, but particularly in the case of a high-rotational-speedmagnetic disk device, the effect of using the magnetic disk substrateaccording to this embodiment remarkably appears as compared with that ofusing a magnetic disk substrate of other than this invention.

The magnetic disk substrate according to this embodiment is preferablyused as a glass substrate to be mounted in a magnetic disk device inwhich a magnetic head scans an offset portion 14 present at theperiphery of the glass substrate at a linear velocity of 20.0 m/sec ormore.

The magnetic disk substrate according to this embodiment is preferablyused for a magnetic disk with a touch-down height (TDH) of 3 to 4 nm orless. If the touch-down height is low, a crash tends to occur when thechange of offset formed on the glass substrate is large. However, usingthe magnetic disk substrate of this embodiment, the change of an offsetportion 14 can be further reduced than conventional and, thus, even ifthe flying height of a magnetic head (recording head) is reduced, it ispossible to prevent the magnetic head from being crashed.

The magnetic disk substrate according to this embodiment is preferablyused for a magnetic disk having a high recording density of 200GBit/inch² or more and more preferably 250 GBit/inch² or more. In thecase of such a high recording density, the flying height of a recordinghead can be further reduced and thus a crash can be prevented by the useof the magnetic disk substrate of this embodiment. It is needless to saythat the magnetic disk substrate according to this invention can also besuitably applied to a glass substrate for use in a magnetic disk havinga recording density smaller than the above or to a magnetic disksubstrate for manufacturing a magnetic disk with a touch-down heightgreater than the above.

The magnetic disk substrate according to this embodiment is adisk-shaped magnetic disk substrate having a flat main surface, an endface, and a chamfered face interposed between the main surface and theend face, wherein, at a peripheral portion of the main surface, thereexists an offset portion 14 offset in a direction perpendicular to aflat surface, excluding the peripheral portion, of the main surface andwherein, in plan view of the main surface of the glass substrate, amaximum point (extreme portion 15), where the degree of offset ismaximum, of the offset portion 14 may be located at positions withapproximately the same distance from a central portion of the glasssubstrate.

In plan view of the main surface of the glass substrate, the maximumpoint of the offset portion 14 may be present in a range of 92.0 to97.0% with respect to the distance to the external of the glasssubstrate from the center of the glass substrate as a reference.

In plan view of the main surface of the glass substrate, the extremeportion 15 of the raised portion circularly surrounds the main surfaceand the circularity of a circle formed by the extreme portion 15 ispreferably 600 μm or less.

The glass substrate has a circular hole at its central portion and, inplan view of the main surface of the glass substrate, the extremeportion 15 of the raised portion circularly surrounds the main surfaceand the concentricity between a circle formed by the extreme portion 15and a circle formed by the circular hole at the central portion ispreferably 1200 μm or less.

The magnetic disk substrate according to this embodiment is adisk-shaped glass substrate having a flat main surface, an end face, anda chamfered face interposed between the main surface and the end face,wherein, in plan view of the main surface of the glass substrate, alowered portion lowered with respect to the main surface is providedbetween the main surface and the chamfered face and the depth of thelowered portion from the main surface may be approximately uniform atpositions spaced from the center of the glass substrate by apredetermined distance.

In the magnetic disk substrate according to this embodiment, it ispreferable that at least the end face be chemically strengthened and itis more preferable that all the surfaces of the substrate be chemicallystrengthened. In other words, in the magnetic disk substrate, it is morepreferable that a compressive stress layer be formed at the surfacesthereof. Particularly, when the magnetic disk substrate is incorporatedinto a magnetic disk device adapted to rotate at high-speed rotation(e.g. 10000 rpm) or into a magnetic disk device for use in a mobileapplication, the impact resistance is required for the glass substrateand thus it is preferable that the compressive stress layer be formed onthe surfaces of the glass substrate. Herein, the chemical strengtheningis a treatment for strengthening a glass substrate by bringing the glasssubstrate into contact with a chemical strengthening treatment solutioncontaining a chemical strengthening salt to thereby exchange part ofions contained in the glass substrate for ions in the chemicalstrengthening treatment solution having an ionic radius greater thanthat of the former ions.

(Magnetic Disk)

Then, by forming a magnetic film on the above-mentioned magnetic disksubstrate, it is possible to manufacture a magnetic disk according tothis embodiment. Since the magnetic film is formed on the magnetic disksubstrate, the shape of a main surface of the magnetic disk is largelyaffected by the glass substrate. That is, in order to improve the shapeof the main surface of the magnetic disk, it is necessary to improve theshape of the main surface of the glass substrate. Therefore, using themagnetic disk substrate disclosed in this embodiment, it is possible tomanufacture the magnetic disk improved particularly in shape of aperipheral portion of the main surface. Since a magnetic diskmanufacturing method is known, a description thereof is omitted herein.

The magnetic disk according to this embodiment may be a magnetic disk tobe mounted in a magnetic disk device of the load/unload type in which amagnetic head is loaded and unloaded with respect to the main surfacethrough its outer periphery. Since the flatness of the outer peripheralportion of the glass substrate is high, the magnetic disk can besuitable for the load/unload type.

The magnetic disk according to this embodiment may be a magnetic disk tobe mounted in a magnetic disk device adapted to rotate it at arotational speed of at least 5400 rpm or more. Further, it can besuitably used even in a magnetic disk device with a speed of 10000 rpmor more. This is because since the flatness of the outer peripheralportion of the glass substrate is high, there is no possibility ofcontact between the magnetic disk and a magnetic head even in the caseof high-speed rotation and thus the reliability is high.

The magnetic disk according to this embodiment may be a contact-slidingtype recording medium (contact-recording type recording medium). In thecase of the contact-sliding type recording medium, a recording headreads and writes in a state where it is in sliding contact with amagnetic disk and, therefore, by improving the flatness, in thecircumferential direction, of particularly the outer peripheral portionof the magnetic disk substrate as described above, it is possible toprevent jumping of the recording head. This makes it possible to improvethe signal quality and to prevent damage to the recording head.

(Magnetic Disk Device)

Then, by mounting the above-mentioned magnetic disk, it is possible toform a magnetic disk device (hard disk drive). The magnetic disk devicemounted with the above-mentioned magnetic disk substrate is particularlysuitable when recording/reproducing information particularly athigh-speed rotation.

Although the description is being given centering on the outerperipheral end face of the magnetic disk substrate, an offset portion 14is also formed with respect to the inner peripheral end face in themanner as described above. It is needless to say that an extreme portion15 where rising in the offset portion 14 is maximum is preferablylocated at approximately the same distance from the center of the glasssubstrate.

Example

Hereinbelow, a description will be given of an Example about methods ofmanufacturing a magnetic disk substrate and a magnetic disk to whichthis invention is applied. These magnetic disk substrate and magneticdisk are manufactured as a magnetic disk having a predetermined shape,such as a 3.5-inch disk (φ89 mm), a 2.5-inch disk (φ65 mm), a 1.0-inchdisk (φ27.4 mm), a 0.8-inch disk (φ21.6 mm), or a 1.8-inch magnetic disk(φ48 mm).

(1) Shaping Process and First Lapping Process

In the magnetic disk substrate manufacturing method according to thisExample, first, lapping (grinding) is applied to surfaces of aplate-like glass to obtain a glass base member, then this glass basemember is cut, thereby cutting out a glass disk. As the plate-likeglass, one of various plate-like glasses can be used. This plate-likeglass can be manufactured, for example, by a known manufacturing method,such as a pressing method, a float method, a downdraw method, aredrawing method, or a fusion method, using a molten glass as amaterial. Among them, if the pressing method is used, the plate-likeglass can be manufactured at low cost. As a material property of theplate-like glass, use can be made of an amorphous glass or a glassceramic (crystallized glass). As a material of the plate-like glass, usecan be made of an aluminosilicate glass, a sodalime glass, aborosilicate glass, or the like. Particularly as the amorphous glass,the aluminosilicate glass can be preferably used in terms of capabilityof being chemically strengthened and capability of providing a magneticdisk substrate excellent in main surface flatness and substratestrength.

In this Example, a molten aluminosilicate glass was molded into a diskshape by direct pressing using upper, lower, and drum molds, therebyobtaining an amorphous plate-like glass. As the aluminosilicate glass,use was made of a glass for chemical strengthening which contains, asmain components, SiO₂: 58 to 75 wt %, Al₂O₃: 5 to 23 wt %, Li₂O: 3 to 10wt %, and Na₂O: 4 to 13 wt %.

Then, lapping was applied to both main surfaces of the plate-like glass,thereby obtaining a disk-shaped glass base member. The lapping wascarried out using a double-side lapping machine employing a planetarygear mechanism with the use of alumina-based free abrasive grains.Specifically, the lapping was carried out by pressing lapping surfaceplates onto both surfaces of the plate-like glass from the upper andlower sides, supplying a grinding liquid containing the free abrasivegrains onto the main surfaces of the plate-like glass, and relativelymoving the plate-like glass and the surface plates to each other. Bythis lapping, there was obtained the glass base member having the flatmain surfaces.

(2) Cutting-Out Process (Coring, Forming, Chamfering)

Then, the glass base member was cut using a diamond cutter, therebycutting out a disk-shaped glass substrate from the glass base member.Then, using a cylindrical diamond drill, an inner hole was formed at acenter portion of the glass substrate, thereby obtaining an annularglass substrate (coring). Then, grinding was applied to an innerperipheral end face and an outer peripheral end face using diamondgrindstones, thereby carrying out predetermined chamfering (forming,chamfering).

(3) Second Lapping Process

Then, second lapping was applied to both main surfaces of the obtainedglass substrate in the same manner as in the first lapping process. Byperforming this second lapping process, fine irregularities formed onthe main surfaces in the cutting-out process or an end face polishingprocess as a preceding process can be removed in advance, so that itbecomes possible to complete a subsequent main surface polishing processin a short time.

(4) End Face Polishing Process

Then, the outer and inner peripheral end faces of the glass substratewere mirror-polished by a brush polishing method. In this event, aspolishing abrasive grains, use was made of a slurry (free abrasivegrains) containing cerium oxide abrasive grains.

Then, the glass substrate having been subjected to the end facepolishing process was washed with water. By this end face polishingprocess, the end faces of the glass substrate were finished to a mirrorsurface state that can prevent precipitation of sodium and potassium.

(5) Main Surface Polishing Process

A first polishing process was first carried out as a main surfacepolishing process. This first polishing process mainly aims to removecracks or strains remaining on the main surfaces during theabove-mentioned lapping processes. In this first polishing process, themain surfaces were polished using a double-side polishing machine havinga planetary gear mechanism with the use of a hard resin polisher. Ceriumoxide abrasive grains were used as a polishing liquid.

The glass substrate having been subjected to the first polishing processwas immersed in respective cleaning baths of neutral detergent, purewater, and IPA (isopropyl alcohol) in turn so as to be cleaned.

Then, a second polishing process was carried out as a main surfacepolishing process. This second polishing process aims to finish the mainsurfaces to a mirror surface state. In this second polishing process,the main surfaces were mirror-polished using a double-side polishingmachine having a planetary gear mechanism with the use of a soft resinfoam polisher. As a polishing liquid, use was made of cerium oxideabrasive grains finer than the cerium oxide abrasive grains used in thefirst polishing process.

The glass substrate having been subjected to the second polishingprocess was immersed in respective cleaning baths of neutral detergent,pure water, and IPA (isopropyl alcohol) in turn so as to be cleaned. Anultrasonic wave was applied to each cleaning bath.

(6) Chemical Strengthening Process

Then, chemical strengthening was applied to the glass substrate havingbeen subjected to the above-mentioned lapping processes and polishingprocesses. The chemical strengthening was carried out by preparing achemical strengthening solution in the form of a mixture of potassiumnitrate (60%) and sodium nitrate (40%), heating this chemicalstrengthening solution to 400° C. and preheating the cleaned glasssubstrate to 300° C., and immersing it in the chemical strengtheningsolution for about 3 hours. The immersion was carried out in a statewhere a plurality of glass substrates were placed in a holder so as tobe held at their end faces, thereby enabling the entire surfaces of theglass substrates to be chemically strengthened.

By performing the immersion treatment in the chemical strengtheningsolution as described above, lithium ions and sodium ions in a surfacelayer of the glass substrate are replaced by sodium ions and potassiumions in the chemical strengthening solution, respectively, so that theglass substrate is strengthened. The thickness of a compressive stresslayer formed at the surface layer of the glass substrate was about 100μm to 200 μm.

The glass substrate having been subjected to the chemical strengtheningtreatment was immersed in a water bath at 20° C. so as to be rapidlycooled, and maintained for about 10 minutes. Then, the rapidly cooledglass substrate was immersed in concentrated sulfuric acid heated toabout 40° C., so as to be cleaned. Further, the glass substrate havingbeen subjected to the sulfuric acid cleaning was immersed in respectivecleaning baths of pure water and IPA in turn so as to be cleaned.

By performing the first lapping process, the cutting-out process, thesecond lapping process, the end face polishing process, the first andsecond polishing processes, and the chemical strengthening process asdescribed above, there was obtained a flat, smooth, and high-rigiditymagnetic-disk substrate.

(7) Inspection Process

An inspection was conducted about the shape of an outer peripheralportion of the obtained magnetic disk substrate. An inspection processcomprises a measurement process of measuring offset values and anextreme portion 15 at a plurality of positions in the circumferentialdirection of the glass substrate and a judgment process of judgingwhether the glass substrate is good or not based on the measured offsetvalues and extreme portion 15.

(8) Magnetic Disk Manufacturing Process

On each of both surfaces of the glass substrate obtained through theabove-mentioned processes, an adhesive layer of a Cr alloy, a softmagnetic layer of a CoTaZr-group alloy, an underlayer of Ru, aperpendicular magnetic recording layer of a CoCrPt-group alloy, aprotective layer of hydrocarbon, and a lubricating layer ofperfluoropolyether were formed in this order, thereby manufacturing aperpendicular magnetic recording disk. This structure is one example ofthe structure of a perpendicular magnetic disk (PMR: PerpendicularMagnetic Recording), while, magnetic layers and so on may be formed as ahorizontal magnetic disk (LMR: Longitudinal Magnetic Recording). Thismakes it possible to manufacture a magnetic disk having high-levelflatness also at an outer peripheral portion of a main surface thereof.

(9) Magnetic Disk Device Manufacturing Process

By incorporating the above-mentioned magnetic disk into a device, amagnetic disk device was manufactured. Since the structure of a magneticdisk device is known, a detailed description thereof is omitted herein.

Example 3

By applying the following polishing conditions to the second polishingprocess of (5) Main Surface Polishing Process described above, amagnetic disk substrate, a magnetic disk, and a magnetic disk devicewere manufactured. In this Example 3, a 2.5-inch disk (φ65 mm) wasmanufactured. The specific polishing conditions in this Example 3 weresuch that, like in the case of the above-mentioned Example 1, thehardness of polishing pads was set to 85 (Asker C hardness), the grainsize of abrasives to 1.0(μm), the processing rate to 0.30(μm/min), andthe processing pressure to 9(Pa). (That is, Example 3 in the secondembodiment of this invention was described as Example 1 in the firstembodiment of this invention.)

Comparative Example 4

A magnetic disk substrate, a magnetic disk, and a magnetic disk deviceaccording to Comparative Example 4 were manufactured by theabove-mentioned manufacturing method except that the polishingconditions of the second polishing process were set to the followingconditions. The specific polishing conditions in Comparative Example 4were such that, like in the case of the above-mentioned ComparativeExample 2, the processing rate was set to 0.45(μm/min) and theprocessing pressure to 8.0(Pa). (That is, Comparative Example 4 wasdescribed as Comparative Example 2 in the first embodiment of thisinvention.)

Comparison between Example 3 and Comparative Example 4

The shape of an extreme portion 15 present at the periphery of a mainsurface of each of the magnetic disk substrates manufactured as shown inExample 3 and Comparative Example 4 was inspected by the followingmethod.

(A) Specification of Surface Shape

In order to examine the shape (end portion shape) of the periphery ofthe main surface of the magnetic disk substrate, an optical interferencesurface shape measuring apparatus (OptiFlat (manufactured by Phase ShiftTechnology, Inc.)) was used. As a result, it was found that the endportion shape of the glass substrates of Example 3 and ComparativeExample 4 was a ski-jump shape. And from this measurement result, thedistance of the vertex (extreme portion 15) in the ski-jump shape fromthe center of the glass substrate was measured. Further, in order toexamine the displacement of the position of the vertex in thecircumferential direction, the position of the vertex was measured at 12points in total by rotating the glass substrate per 30° in thecircumferential direction. The results in this event are shown in FIG.6. FIG. 6 is a diagram showing the results of measuring the radialposition of the extreme portion (ski-jump point) 15 of the ski jump.

As a result, as shown in FIG. 6, it was found that the extreme portion15 formed at the periphery of the main surface of the magnetic disksubstrate according to Example 3 was located at approximately the sameposition (distance) as seen from the center of the glass substrate and,specifically, was located in a range of ±0.2 mm with respect to 30.6 mmfrom the center of the glass substrate. On the other hand, in the caseof Comparative Example 4, it was found that the extreme portion 15 waslocated in a range of ±1.4 mm with respect to 30.6 mm.

The circularity of the magnetic disk substrate according to Example 3was 0.40(mm) and the concentricity thereof was 1.07(mm). On the otherhand, the circularity of the magnetic disk substrate according toComparative Example 4 was 2.60(mm) and the concentricity thereof was5.68(mm). With respect to the height (offset value) of the extremeportion 15, the values were approximately the same in the structures ofExample 3 and Comparative Example 4.

(B) Load/Unload Test Comparison

After manufacturing the magnetic disks in which a magnetic layer wasformed on the magnetic disk substrates according to Example 3 andComparative Example 4, the magnetic disk devices were manufactured, anda load/unload test was conducted in which the rotational speed was setto 10000 rpm. In this event, the flying height of a recording head wasset to 9 to 10 nm. As a result, no crash occurred even by repeating theload/unload 1000000 times in the case of Example 3, while, in the caseof Comparative Example 4, a crash occurred when the load/unload wasrepeated 600000 times. In Example 3, in the case of a rotational speedof 5400 rpm and 7200 rpm, no crash occurred even by repeating theload/unload 1000000 times.

Further, despite the offset values were approximately the same inExample 3 and Comparative Example 4, a crash failure occurred with thestructure of Comparative Example 4 when a glide test was carried out athigh-speed rotation. On the other hand, no crash failure occurred withthe structure of the Example. This is presumably because even if large(not small) ski jump is present, since the circularity or concentricityis high (value is small), the change (roughness or undulation) of thesubstrate surface in the circumferential direction is small. Further, asa result of many tests, it was found that the circularity was preferably600 μm or less and the concentricity was preferably 1200 μm or less.

From this result, it is seen to be important that, as in this invention,the radial position of the extreme portion (vertex of the raisedportion) 15 of the offset portion 14 present at the periphery of themain surface of the glass substrate be approximately the same in thecircumferential direction.

(C) Influence of End Portion Shape

Next, the influence of the height of the extreme portion 15 in thecircumferential direction was examined. An examination was made of theinfluence of the change in height of the vertex, in the circumferentialdirection, of the magnetic disk substrate in which the radial positionof the extreme portion (vertex of the raised portion) 15 of the offsetportion 14 present at the periphery of the main surface of the glasssubstrate is approximately the same in the circumferential direction(Example 3 described above). Further, another magnetic disk substrate inwhich the change in height of the vertex in the circumferentialdirection is greater than that of the above-mentioned Example 3 wasprepared as Comparative Example 5. This magnetic disk substrate ofComparative Example 5 was manufactured by setting the processingpressure and the processing rate in the final polishing process to bedifferent from those in the above-mentioned Example 3. Specifically, themagnetic disk substrate of Comparative Example 5 was manufactured bysetting the hardness of polishing pads to 85 (Asker C hardness), thegrain size of abrasives to 1.0(μm), the processing rate to 0.60(μm/min),and the processing pressure to 12.0(Pa) like in the case of theabove-mentioned Comparative Example 1. (That is, Comparative Example 5was described as Comparative Example 1 in the first embodiment of thisinvention.)

Then, in order to examine the change in height of each glass substratein the circumferential direction, the maximum offset value of eachsubstrate between two arbitrary points was measured. Specifically, themeasurement range (R1, R2 in FIG. 2) of the maximum offset value wasdetermined so that it was possible to see the change of the extremeportion 15 formed at the periphery of the main surface of the glasssubstrate and having heights that differ in the circumferentialdirection. Herein, the distances from the center of the substrate wereset to 29.9 mm (R1) and 31.5 mm (R2), respectively, and, as a measuringapparatus, use was made of an optical interference surface shapemeasuring apparatus (MicroXam (manufactured by Phase Shift Technology,Inc.)). Then, the position of the vertex was measured at 12 points intotal by rotating the glass substrate per 30° in the circumferentialdirection. MicroXam is narrow in measurement range but high inresolution as compared with OptiFlat described above.

That is, the maximum offset value measured by the above-mentioned methodrepresents the magnitude of the offset between the above-mentionedstraight line connecting R1 and R2 and the vertex (extreme portion 15)of the above-mentioned raised portion. The results are shown in FIG. 7.

As shown in FIG. 7, the change in height of the glass substrate in thecircumferential direction is small in Example 3 as compared with that inComparative Example 5. Specifically, the change difference in Example 3was 2.86 nm, while, the change difference in Comparative Example 5 was16.10 nm.

After manufacturing the magnetic disks in which a magnetic layer wasformed on the magnetic disk substrates according to Example 3 andComparative Example 4, the magnetic disk devices were manufactured, anda load/unload test was conducted. Specifically, the test was carried outin two cases with a disk rotational speed of 5400 rpm and 7200 rpm bysetting the flying height of a recording head to 9 to 10 nm.

As a result, in the case of the magnetic disks according to Example 3and Comparative Example 4, no crash occurred even by repeating theload/unload 1000000 times at a rotational speed of 5400 rpm. Withrespect to the magnetic disk of Comparative Example 5, a crash occurredin a load/unload test of 2000000 times.

From this result, it is seen to be most preferable in terms of rotatingthe magnetic disk at high speed that the extreme portion 15 be the samein radial position and small in height change in the circumferentialdirection of the glass substrate.

While the second preferred embodiment of this invention has beendescribed with reference to the accompanying drawings, it is needless tosay that this invention is not limited thereto. It is obvious that aperson skilled in the art can think of various modified examples orrevised examples within the scope described in claims and it isunderstood that those naturally also belong to the technical scope ofthis invention.

For example, it has been described in the above-mentioned Example thatthe substrate according to this invention is a glass (amorphous glass orglass ceramic (crystallized glass) can be used; as a material of aplate-like glass, aluminosilicate glass, sodalime glass, borosilicateglass, or the like). However, since this invention relates to the shapeof a substrate, this invention is not limited to a material of thesubstrate and thus can be suitably applied, for example, even to asubstrate made of aluminum or a material other than it. As describedabove, however, a glass substrate excellent in substrate surfaceflatness and substrate strength as compared with an aluminum substrateis preferable particularly for a mobile device.

The magnetic disk substrate according to this invention is a magneticdisk substrate having a generally flat main surface, an end face, and achamfered face formed between the main surface and the end face and maybe configured such that, at the periphery of the main surface, there isprovided an offset portion 14 raised or lowered with respect to a flatsurface, other than the periphery, of the main surface, the offsetportion 14 is continuously formed in the circumferential direction ofthe main surface, and the position of an extreme portion 15 where risingis maximum in the offset portion 14 is located at approximately the samedistance from the center of the substrate.

The offset portion 14 may be formed at the outer periphery or the innerperiphery of a main surface of a disk-shaped magnetic disk formed withan inner hole in its center or may be formed at each of them. In thisinvention, when the offset portion 14 is formed at each of the innerperiphery and the outer periphery, at least one of the offset portions14 may have the above-mentioned shape, but it is preferable thatparticularly the outer periphery where the linear velocity of arecording head is high have the above-mentioned shape.

INDUSTRIAL APPLICABILITY

This invention can be used as a magnetic disk substrate for use in amagnetic recording medium and as a magnetic disk using it.

1. A magnetic disk substrate being a disk-shaped substrate and having amain surface, an end face, a chamfered face formed between the mainsurface and the end face, and an offset portion, at a periphery of themain surface, formed with offset raised or lowered with respect to aflat surface, other than the periphery, of the main surface, wherein: ashape of said offset portion is uniform over the entire circumference ofsaid substrate.
 2. A magnetic disk substrate being a disk-shapedsubstrate having a generally flat main surface, an end face, and achamfered face interposed between the main surface and the end face,wherein: a raised portion raised with respect to the main surface isprovided between the main surface and the chamfered face, and wherein:in plan view of the main surface of said disk-shaped substrate, saidraised portion surrounds said main surface with an approximately uniformheight.
 3. A magnetic disk substrate being a disk-shaped substratehaving a generally flat main surface, an end face, and a chamfered faceinterposed between the main surface and the end face, wherein: in planview of the main surface of said disk-shaped substrate, a loweredportion lowered with respect to said main surface is formed in acircular shape between said main surface and said chamfered face and adepth of said lowered portion from said main surface is approximatelyuniform at positions spaced from a center of said disk-shaped substrateby a predetermined distance.
 4. A magnetic disk substrate according toany one of claim 1, 2, or 3, characterized by being a substrate for useas a substrate of a magnetic disk adapted for a DFH (dynamic flyingheight) head.
 5. A magnetic disk substrate according to any one of claim1, 2, or 3, wherein: said substrate is a glass substrate capable ofbeing chemically strengthened and having an ion-exchanged layer at leasta part of the surface and faces of said substrate, and a thickness ofsaid ion-exchanged layer is greater at the end face than at the mainsurface.
 6. A magnetic disk substrate being a disk-shaped substrate andhaving a generally flat main surface, an end face, a chamfered faceformed between the main surface and the end face, and an offset portion,at a periphery of the main surface, raised with respect to a flatsurface, other than the periphery, of the main surface, wherein: saidoffset portion is formed over the entire circumference of saidsubstrate, and an extreme portion where rising is maximum in said offsetportion is located at approximately the same distance from a center ofsaid substrate.
 7. A magnetic disk substrate according to claim 6,wherein said extreme portion in the raised offset portion is locatedover the entire circumference of said offset portion in a range of 92.0to 97.0% with respect to a radial distance from the center of saidsubstrate to its outer diameter.
 8. A magnetic disk substrate accordingto claim 6, wherein, in said main surface, a circularity of a circleformed by said extreme portion is 600 μm or less.
 9. A magnetic disksubstrate according to claim 6, wherein: said substrate has a circularhole in its center, and in said main surface, a concentricity between acircle formed by said extreme portion and said circular hole is 1200 μmor less.
 10. A magnetic disk substrate according to claim 6, wherein:said offset portion includes rising and/or lowering with respect to theflat surface, other than the periphery, of said main surface, saidrising and/or lowering formed at the periphery of said main surface, anda magnitude of offset in said offset portion is approximately uniformover the entire circumference of said substrate.
 11. A magnetic disksubstrate according to claim 10, wherein, in said offset portion, achange in magnitude of said offset is 5 nm or less.
 12. A magnetic disksubstrate according to claim 6, wherein said substrate is a substratefor use in a magnetic disk to be mounted in a magnetic disk device of aload/unload type in which a magnetic head is loaded and unloaded withrespect to a main surface of the magnetic disk through its outerperiphery.
 13. A magnetic disk substrate according to claim 6, whereinsaid substrate is a substrate for use in a magnetic disk to be mountedin a magnetic disk device adapted to rotate the magnetic disk at arotational speed of at least 5400 rpm or more.
 14. A magnetic diskwherein at least a magnetic layer is formed on the substrate accordingto claim
 6. 15. A magnetic disk according to claim 14, wherein atouch-down height is 4 nm or less.
 16. A magnetic disk according toclaim 14, wherein a recording density is 200 GBit/inch2 or more.
 17. Amagnetic disk device mounted with the magnetic disk according to claim14.